The most unpredictable election campaign in modern times moves chaotically towards its close, with David Cameron "ambushed" by the parent of a disabled child, and Gordon Brown overheard disparaging one woman voter as a "bigot". But bigger issues are at stake. The uncertainty over the outcome offers a historic opportunity for progressive, centre-left politics - but it also presents a substantial danger. A sense of transformative change has become the overwhelming theme of this election. The numerous permutations in a hung parliament are perpetually discussed. However, the question of what would happen if one party wins an overall majority is seldom raised, as if the outcome were already known.

It is possible that the final result of the election will break the mould of British politics and alter the landscape permanently. This could be the last election fought under the outdated and absurdly unfair first-past-the-post voting system. The stifling dominance at national level of two increasingly sclerotic parties could be ended. Landslide governments that are able to ignore parliament while remaining neurotically fearful of the media might be consigned to history, allowing a more robust House of Commons to hold administrations to account. And not a moment too soon. As Peter Wilby argues on page 14, newspapers have long colluded with politicians in maintaining the two-party system.

Perhaps we are on the edge of history. But perhaps we are not. The excitement generated in the campaign does not itself change very much at all. The surge in support for the Liberal Democrats certainly reflects more than a superficial discontent with both the main parties. Yet the deeper reasons for "Cleggmania" are unclear. Most of the converts are unlikely to know a great deal about Lib Dem policies. Few will have examined in any detail the likely consequences of a hung parliament, or even how the last one worked after the election of February 1974. What we are witnessing is, in part, one long, cathartic scream of anger after a severe economic recession and a sordid parliamentary scandal. No one knows what will happen next. The last time similar excitement gripped British politics was in the early 1980s, with the formation of the Social Democratic Party (SDP).

At first, the Conservatives appeared to be the chief victim of SDP-mania. In 1981, at the peak of the SDP's popularity, the Times declared Margaret Thatcher "the most unpopular Prime Minister of the century". However, she won a landslide victory less than two years later. As it turned out, disillusioned Labour voters were more inclined to vote for the new party than Conservative supporters: the centre-left vote was split in 1983, and again in 1987.

There is no possibility that Mr Cameron's Conservatives will achieve the levels of support that Mrs Thatcher enjoyed. But there are signs that it is Labour voters who have been most intoxicated by the rise of Nick Clegg. This is the danger. With the surge in support for the Lib Dems, the main battlegrounds are now those marginal seats in which Labour is fighting it out with the Conservatives: constituencies where Mr Clegg's party cannot win. Senior Labour strategists fear that enough of their supporters will switch to the Lib Dems in these seats to produce substantial Tory gains. Consequently, "Cleggmania" could let Mr Cameron in to No 10 through the back door, in the same way that the SDP inadvertently delivered two landslides for Mrs Thatcher. Moreover, as Peter Kellner writes on page 20, even if the election does result in a hung parliament, constitutional precedent favours the opposition rather than the government. That, indeed, is one of the lessons of February 1974, when Harold Wilson returned as prime minister after Edward Heath was unable to reach a deal with Jeremy Thorpe's Liberals.

If Mr Cameron does manage to win even a small overall majority, there will be no change to the voting system. George Osborne will introduce his emergency Budget and begin the premature and punitive cuts in spending that could send Britain back into recession. Labour would form the main opposition because, barring the most sensational outcome on polling day, it will win many more seats than the Liberal Democrats. In short, the two-party duopoly will stay.

Labour is clearly no longer the sole vehicle of progressive politics in this country. But only a progressive realignment that brings together Labour and the Lib Dems can deliver the fundamental economic and constitutional reforms that the country needs. The bleak alternative is an unreconstructed Tory government dedicated to preserving the status quo.