In Britain, peacetime coalitions have been based largely on fear - fear of Irish Home Rule in 1895, of "Bolshevism" and revolution in 1918 and of financial collapse in 1931. Coalitions held together as long as fear predominated. When the fear disappeared, the rationale for the coalition also went.
What is remarkable about the Lloyd George coalition is how rapidly it came to an end, just four years after its landslide victory in the 1918 election. "He can be dictator for life if he wishes," Bonar Law, the Conservative leader, had declared in 1918. But Lloyd George was discredited by the horrors of the Black and Tans in Ireland and the sale of honours. A wag declared that the coalition had become a deal between a flock of sheep led by a crook (the Liberals) and a flock of crooks led by a sheep (the Conservatives). In 1922, the coalition came to an end and Lloyd George never held office again.
The National Government lost the support of the Liberals, its one properly independent non-Conservative element, in 1932, just a year after winning the largest electoral landslide in British history in 1931.
In both cases, the revolt that undermined the coalition came not from the leadership, but from the grass roots. Historians have emphasised the meeting of Conservative MPs at the Carlton Club in 1922 which voted to end the Lloyd George coalition. But the coalition was doomed whatever MPs thought. It was being repudiated by Conservative constituency associations, which were adopting candidates opposed to its continuation. By the time of the Carlton Club meeting, over 180 such candidates had been selected.
Snakes in the grass
In 1932, Liberal activists pressed their leaders to leave the National Government. The 1932 conference of the National Liberal Federation condemned the "agreement to differ" by which Liberals sought to remain in the Conservative-led government, despite its departure from free trade. The former party leader Lloyd George, who had opposed the National Government, declared that the undignified position of the Liberals resembled that of a cat that "has pushed its head into a cream jug and cannot get it out without either breaking the jug or having someone pull it out by the tail. It is the latter process that is going on at the moment and I hope it will succeed."
It is the grass roots that threaten the 2010 coalition, not conflicts among the leaders. The coalition, it has been said, comprises four parties, not two. It is buttressed by the Cameroons and the Orange Book Liberal Democrats, but the other two elements, the Conservative right and the progressive Liberal Democrats, remain distinctly sceptical.
The by-election in Oldham East and Saddleworth shows that progressive Liberal Democrats are switching to Labour. Admittedly, this switch was compensated by Conservative voters switching tactically to the Liberal Democrats. But this was at the cost of alienating Conservative Party members, the local activists who keep the wheels turning. Why should they continue to support party leaders who wish their Liberal Democrat opponents well?
The history of coalitions shows how difficult it is, under the first-past-the-post electoral system, for parties co-operating in government to avoid conflict in the constituencies. Constituency parties still enjoy considerable autonomy. They cannot be compelled by party HQs not to put up a candidate, and voters cannot be ordered to transfer their allegiance. When the next general election comes, some Conservatives, rather than supporting Liberal Democrats, might support Ukip, and many Liberal Democrats will vote for Labour or the Greens.
The coalition will face further tensions during the elections to the Scottish Parliament in May. The general election, in which the Conservatives won just one seat in Scotland, shows that they remain toxic north of the border, and the Conservative unpopularity could rub off on the Liberal Democrats. Even so, the outcome might well be a Labour/Liberal Democrat coalition in Holyrood, and Scottish Lib Dems might then put pressure on MPs and ministers to leave the coalition.
The reduction in the number of MPs in the Voting System and Constituencies Bill, together with the 2013 electoral review, implies a wholesale redrawing of boundaries. Every constituency will be choosing its candidate anew. Both Conservative and Liberal Democrat constituency associations may prefer candidates pledged to their party rather than to the coalition.
Paradoxically, the coalition would benefit from a Yes in the referendum on the Alternative Vote (AV), which David Cameron opposes but Ed Miliband supports. AV would allow co-operation at constituency level without withdrawal of candidates. Conservatives would ask their supporters to give their second preferences to Liberal Democrats, while Liberal Democrats would do the same for Conservatives. It does not, of course, follow that voters would accept this advice.
Birds of paradise
Asked the likely outcome of the Carlton Club meeting in 1922, a prominent Tory replied: "A slice off the top." What he meant was that Conservatives at the grass roots had already decided to end the coalition. The only issue was whether the leaders would respect that decision; if they did not, they would be repudiated. Commentators overestimated the stability of the Lloyd George coalition because they focused on its "glittering birds of paradise", in Lord Beaverbrook's phrase - Lloyd George, Churchill and F E Smith - rather than the less glamorous party workers who opposed it. Perhaps they are making the same mistake today.
The 2010 coalition, like its predecessors, was based in part on fear - fear that Britain was in danger of a Greek-style financial collapse. When that fear disappears, the coalition will be held together primarily by the desire of the two parties to cling to office. But peacetime coalitions collapse through disaffection at the grass roots, not conflict at the top. In Britain, the leaders can only lead for as long as the followers are willing to follow. When the followers cease to follow, the leaders cease to be able to lead.
The challenge for Miliband is to unite progressive forces, at present scattered, but probably still comprising a majority of the electorate.
Vernon Bogdanor is research professor at the Institute of Contemporary History, King's College London. His book "The Coalition and the Constitution" will be published in March.