"If I am to die, I would rather die fighting on the left," said David Lloyd George when, in 1931, he explained why he would not be joining most of the Liberal Party in coalition with the Conservative-dominated National Government.

The present coalition between the Conservatives and Liberal Democrats has long ante­cedents. The Conservatives were in coalition with dissident Liberals in 1895, when the Liberal Unionists joined them to resist Home Rule; from 1916 to 1922, when a group of Liberals led by Lloyd George joined with the Conservatives to win the First World War and secure postwar reconstruction; and again, in 1931, in the National Government.

Indeed, it was this government which, in order to retain Liberal support, invented the "agreement to differ", whereby ministers were allowed to retain their own views on the relative merits of free trade or an imperial tariff. This expedient will be copied by the present government if and when there is a referendum on the Alternative Vote system.

But, in 1931, the "agreement to differ" did not work well. The House of Commons faced the interesting spectacle of the Conservative chancellor of the Exchequer, Neville Chamberlain, insisting that a tariff was needed to save the country, while the home secretary, Sir Herbert Samuel, his Liberal colleague, also speaking from the front bench, warned that it represent­ed the road to ruin. The "agreement" did not last. Adopted in January 1931, it collapsed the following August when the Liberals refused to accept the Ottawa agreements extending the tariff, and resigned from the government.

The Liberals have formed coalitions with the Conservatives, but they have never been in a peacetime coalition with Labour. Though it may be a bit much to conjure the ghost of Ramsay MacDonald, as John Reid and David Blunkett did during the fraught period that led to the creation of the new government, it should be rememberered that the Liberals have supported minority Labour governments - in 1924, 1929-31 and during the Lib-Lab pact in 1977-78.

Yet coalition government has always benefited the Conservatives. It enabled them to hold power with the aid of satellite parties, splinters from their opponents such as the Liberal Unionists and Liberal Nationals, at times when they might not have been able to win on their own. Three times in the 20th century they won landslide majorities, which they could scarcely have managed unaided, with support from outsiders - Joseph Chamberlain in 1900, Lloyd George in 1918 and Ramsay MacDonald in 1931.

“The last purely Conservative government," Harold Macmillan wrote in 1975, perhaps with his tongue in his cheek, "was formed by Mr Disraeli in 1874 - it is the fact that we have attracted moderate people of Liberal tradition and thought into our ranks which makes it ­possible to maintain a Conservative government today. A successful party of the right must continue to recruit its strength from the centre, and even from the left centre. Once it begins to shrink into itself like a snail it will be doomed . . ."

In the 1950s, supporters of the Churchill government, such as the "radio doctor" Charles Hill, stood not as Conservatives but under the label "Liberal and Conservative". Others stood as "National Liberals". When the leader of the Liberals, Clement Davies, wrote to Churchill attacking him for subterfuge, the Conservative leader replied, with superb insouciance, that "I should not presume to correct your knowledge of the moral, intellectual and legal aspects of adding a prefix or suffix to the honoured name of Liberal". The last "National Liberal" stood in 1966, after which the party merged with the Conservatives, as the Liberal Unionists had done in 1912.

Coalitions have been of less benefit to the Liberals. In fact, they have always led to, or have been the product of, a party split - for example, with the Liberal Unionists, who split from the Liberals over Home Rule in 1886, and the Liberal Nationals in the 1930s. One wing of the party would subsequently be swallowed up by the Conservatives, while the other wing remained independent. In 1932, when one group of Liberals left the National Government over free trade, the Liberal Nationals (later National Liberals) remained, and, under the leadership of Sir John Simon, became even more enthu­siastic appeasers of Nazi Germany than the Conservatives. A similar split could easily occur today, with the right wing of the Liberal Democrats under David Laws merging with the Conservatives, while the left, under Vince Cable and Simon Hughes, deserts the coalition, perhaps to seek an arrangement with Labour.

There is, however, one fundamental and crucial difference between past Conservative-Liberal coalitions and the present one. In the past, coalitions were formed before a general election and not after it. In 1895, 1918 and 1931, electors knew before they came to vote that the Conservatives and Liberal Unionists, or Lloyd George Liberals, or Liberal Nationals, were in coalition. In 2010, by contrast, voters had to guess what the Liberal Democrats would do in a hung parliament. Most would have guessed wrong.

Nick Clegg was tactically shrewd in not making his preference known; had he done so, the Lib Dems would have secured fewer than the 57 seats they won. But in Oxford West and Abingdon, the seat lost by Dr Evan Harris, leaflets were delivered to electors telling them that voting Liberal Democrat was the only way to keep the Conservatives out, given that Labour, a distant third, had no chance. David Cameron agreed, saying that if one voted on the Thursday for the Lib Dems, one would wake up on the Friday with Gordon Brown. Perhaps the leaflet should have said that voting Liberal Democrat was the only way to keep the Conservatives in; indeed, after the election, the defeated Harris endorsed the coalition with the Conservatives. Perhaps in Labour-Lib Dem marginals, electors were told that voting Liberal Democrat was the only way to keep Labour out, since the Conservatives, a poor third, had no chance? As a campaign guide published by the Association of Liberal Democrat Councillors declared: "You can secure support from voters who normally vote Tory by being effectively anti-Labour and, similarly, in a Tory area secure Labour votes by being anti-Tory."

The Lib Dems were free, therefore, to decide which way they should jump. And it is this that so upsets opponents of proportional representation. Why, they ask, should a party that has won fewer votes than the two "old" parties decide which of them should form a government?

Proportional representation does not invariably magnify the power of a centre party. Some systems, such as the Single Transferable Vote (used for all elections except parliamentary elections in Northern Ireland, for local elections in Scotland, and for all but presidential elections in the Republic of Ireland) and the additional-member system (used in Germany, and now also in Scotland, Wales and London for the election of the devolved bodies), allow voters to indicate, through their preferences or distribution of constituency and list votes, precisely which coalition they would like to see.

A party that seeks to ignore these signals will soon be repudiated by its members. The German Free Democrats - close in ideology to the Lib Dems - have always announced before an election which party they would join in coalition. Before the recent federal elections in Germany, no voter could have been in any doubt that the Free Democrats would join with the Christian Democrats, and not with the ­Social Democrats.

How were Lib Dem voters to ascertain what the party would do in a hung parliament? Looking at such manifesto commitments as an amnesty for illegal immigrants and the non-renewal of Trident, as well as the agreement with Labour that dealing with the deficit should wait until the recovery was assured, it would not have been unreasonable to assume that a vote for the Lib Dems was a vote for a party of the left. The last YouGov survey before the election, on 5 May, found that 43 per cent of Liberal Democrat voters described themselves as centre left or left, as compared with 29 per cent who described themselves as centrist and 9 per cent who described themselves as centre right or right. Moreover, 39 per cent of Lib Dem voters thought the party was left or centre left, as compared with the 33 per cent who thought it was centrist and 5 per cent who thought it was right or centre right. Nick Clegg appeared to agree with them: last year, he wrote a pamphlet entitled The Liberal Moment, in which he argued that "progressive conservatism" would prove a contradiction in terms. A Populus poll for the Times on 8 May revealed that, while a minority Conservative government was the favourite option, attracting 53 per cent support, a Liberal Democrat-Labour arrangement was preferred to a Liberal-Conservative one by 51 per cent to 46. In forming the coalition, the Liberal Demo­crats may have ignored the views both of their members and those who voted for them.

Past Conservative-Liberal coalitions were buttressed by an electoral pact setting out mutual withdrawal of candidates, perhaps the only satisfactory long-term basis for coalition government. These pacts - with the Liberal Unionists from 1886, with the Lloyd George Liberals of 1918 and with the Liberals and Liberal Nationals in 1931 - prefigured long periods of Conservative hegemony. The Cameron-Clegg coalition is unlikely to be underwritten by an electoral pact. So, as in the forthcoming Thirsk and Malton by-election, parties co-operating on common policies in government will be competing at the polls. Nevertheless, the government may survive for longer than many imagine.

The two party leaders have given many high-minded reasons for joining a Conservative-led coalition. But there are some less high-minded motives. The Liberal Democrats have insufficient funds to fight another election, and little stomach for one, fearing, no doubt rightly, that when the voters are asked to make up their minds, they will polarise, and the Lib Dems will be squeezed - which is what happened in 1924 after the first minority Labour government, in 1951 after the short parliament of 1950-51, and in October 1974, after the last hung parliament, when the Liberals lost a million votes in just seven months. The likelihood is that, as Lloyd George predicted of the Balfour Conservative-Liberal Unionist coalition of 1902-1905, the coalition ministers will die with their drawn salaries still in their hands.

The decision by the Lib Dems to form a coalition with the Conservatives brings to an end the project of realignment on the left, begun by Jo Grimond in the 1950s, and continued by David Steel in the 1970s and by Paddy Ashdown, with support from Tony Blair, in the 1990s. This project sought to overcome the split between what were regarded as two parties of the left. By 2010, it had even won the support of the trade unions, who preferred a Labour-Lib Dem "rainbow coalition", however weak, to a Conservative government. The project would have required commitment to a
referendum on proportional representation and perhaps also an electoral pact. It could have ensured the hegemony of the left. But in 1997, Labour was too strong, having just won a landslide. By 2010, it was too weak.

Perhaps the Liberal Democrats have now become not an anti-Conservative party, but an anti-Labour party, as the Liberals were in the years before Grimond, under Clement Davies, party leader from 1945 until 1956. At the 1951 general election, there were local electoral pacts between the Liberals and the Conservatives (without which the latter would probably not have returned to government). Just one of the six Liberal MPs won against Conservative opposition, and that was Grimond himself, in the far-flung constituency of Orkney and Shetland. The Liberal Party had become dependent on the Conservatives for its very existence.

Alternatively, the Lib Dems may have become neither an anti-Conservative nor an anti-Labour party, but a hinge party, willing to trade their position with either of their larger opponents in the hope of eventually achieving their cherished goal of proportional representation. The first step to achieving that end would be the Alternative Vote (AV), which the Lib Dems hope might be a prelude to Alternative Vote-plus (AV+), the system recommended in the Jenkins report of September 1998 (the conclusion of the Independent Commission on the Voting System). But the AV can prove to be an even more disproportional system than first-past-the-post.

AV is a system of preferential voting in single-member constituencies. It ensures that no MP is elected on a split vote. The Conservatives won more seats on a split vote than Labour at every election between 1918 and 1992, but were over-represented only in general elections that they won - in 1983 and 1987, for example. In elections such as 1945 and 1966, which they lost, they were under-represented. AV would, therefore, have made the outcome even more disproportional in 1945 and 1966 by further over-representing Labour. In 1997 and 2001, for a similar reason, Labour would have won a larger majority under AV than under first-past the-post. The system does nothing to diminish the importance of geography in elections.

The referendum on the Alternative Vote will be put forward by a party that is opposed to it - the Conservatives - and by another that hopes to use it to ensure a second change in the electoral system which would bring about some form of proportional representation. That is an odd way to begin the "new politics".

It seems the Labour Party and the left do not yet realise what a catastrophe has hit them. It is comparable to 1983, though then the left could at least hope that Labour and the SDP-Liberal Alliance might come together. There seemed to be an anti-Conservative majority in the country that was frustrated by the electoral system. Today, there seems to be an anti-Labour majority comprising the Conservatives, accounting for 36 per cent of the vote, and the Liberal Democrats, representing 23 per cent.

In 2010, 85 MPs were elected owing no allegiance to either of the major parties. This means that, for Labour to win an election, it needs to win well over 85 seats more than the Conservatives. They have done this just five times since the war - in 1945, 1966, 1997, 2001 and 2005. In 1945 and 1966, however, third-party representation was negligible. Labour's only chance of escaping from opposition in this parliament seems to be to detach some or all of the Lib Dems from the government.

That will be possible only if Labour comes to embrace proportional representation, over the opposition of tribalists such as Jack Straw, Blunkett and Reid. There are some in Labour who say that a short period in opposition would refresh the party. Many said the same in 1951 and 1979. The party was in opposition for 13 and 18 years, respectively.

In a review of a book by Tim Bale on the Conservatives written shortly before the general election, I argued that David Cameron had become the "most formidable threat that the left has faced since the advent of Thatcher". How right I was.

Vernon Bogdanor is professor of government at Oxford University and the author of "The New British Constitution" (Hart, £17.95)