The Liberal Democrat leader Charles Kennedy says that the current electoral system is unfair, and the fact that Labour can govern with such a large majority in Parliament with relatively narrow support among the electorate is bad for democracy.
In their 1997 Manifesto, Labour said that it "was committed to a referendum on the voting system for the House of Commons."
The Jenkins Commission report on electoral reform recommended that 15% to 20% of seats should be elected by proportional representation in order to "reduce the disproportionality and the geographical divisiveness" of the current system without "imposing a coalition habit on the country".
But no action was taken.
Proportional representation has been introduced, however, for a number of other UK elections, including the Scottish Parliament, the Welsh Assembly, the European Parliament, and the London mayor.
In its 2005 manifesto, Labour says that it "remains committed to reviewing the experience of the new electoral systems", but "a referendum remains the right way to agree any change for Westminster".
The Conservatives have no plans to introduce PR for Westminster, but they want an elected House of Lords and a smaller House of Commons.
In the 2001 General Election, Labour won 62% of the seats with 40.7% of the vote. Taking into account those who did not vote, that meant 24.2% of people eligible to vote, voted Labour in 2001.
There are two sources of perceived unfairness in the UK electoral system.
First, the average number of voters in Labour-held seats (66,997) is 5,000 less than the number of voters in Conservative-held seats (72,849).
This is because the election in England and Wales is being fought on the 1997 constituency boundaries, and in the intervening years Labour's (mainly urban) seats have lost population, while the Tory (mainly suburban) seats have gained population. (The next revision -which takes place every 8 to 12 years - will come into effect in 2007, while boundaries have already been revised in Scotland).
And given that turnout is substantially lower on average in Labour seats, it takes far fewer voters to elect a Labour MP than to elect a Conservative MP.
This is why if Labour and the Conservatives both got 36% of the vote (with the Lib Dems on 22%), Labour would have an estimated majority of 64 in the next Parliament on a uniform swing.
Secondly, the first-past-the-post system exaggerates the advantage of the winning party, particularly at the expense of third parties.
For example, if the Lib Dems increased their share of the vote by 50% (from 19% to 28%) at the expense of Labour, and the Conservatives and Labour both won 33%, then on a uniform swing Labour would have 347 seats, a majority of 48, with 191 Conservatives and just 77 Lib Dem seats, according to estimates by Professor John Curtice of Strathclyde University.
If turnout was the same as in 2001, this could give Labour a substantial majority when only one in five of the electorate has voted for them.
The current system also increases the importance of the 100-150 marginal seats where most campaigning is concentrated, and electoral reformers argue that it means the voters in the remaining 400 seats are regarded as irrelevant.
The Electoral Reform Society, which campaigns for PR, says there are 150 Labour seats which they have held continuously for 60 years, and 100 for 70 years.
And they claim that the fact the most votes are in this sense "wasted" contributes to the alienation from the electoral system and low turn-out.
A move towards an electoral system based on full proportional representation has always been rejected by the two biggest parties on the grounds that it would produce weak coalition governments and break the link between MPs and their constituents.
There is no reason to believe they have fundamentally changed their views.
But if the result is very close in the popular vote, such a debate may well revive.
And proportional representation could form part of the plan for further reform of the House of Lords, and is already planned for local government in Scotland.