Nick Clegg will demand that Gordon Brown improves on his "miserable little compromise" of limited electoral reform as the price of propping up Labour in a hung parliament.
In an interview with The Independent, the Liberal Democrat leader rejected Labour's proposals for electoral reform, which stop short of proportional representation (PR), and insisted on a truly proportional system for electing MPs.
Mr Clegg said the latest opinion polls, which suggest Labour could come third in the share of the vote but cling on to power, would make the campaign for PR unstoppable: "It is not going to be a question of us propping up [another party] but of us insisting on the changes only we advocate."
Raising his party's sights, he declared: "Everyone says the only question is whether we could support another party. But I think it is now much more open than this. We are going for broke. I want to try to push this all the way."
Until now, the Liberal Democrats have suggested they would accept the alternative vote (AV), with people listing candidates in order of preference, on which Labour has promised a referendum next year. But Mr Clegg is now demanding the "alternative vote plus" system, which unlike AV is proportional and was recommended by Lord Jenkins of Hillhead in 1998.
Mr Clegg said: "AV is a baby step in the right direction – only because nothing can be worse than the status quo. If we want to change British politics once and for all, we have got to have a quite simple system in which everyone's votes count. We think AV-plus is a feasible way to proceed. At least it is proportional – and it retains a constituency link.
"The Labour Party assumes that changes to the electoral system are like crumbs for the Liberal Democrats from the Labour table. I am not going to settle for a miserable little compromise thrashed out by the Labour Party."
Mr Clegg suggested he would not force Mr Brown to stand down as the price of a deal with Labour. "If we can deliver a once-in-a generation opportunity to change politics from top to toe, I am not going to start being precious about my personal likes and dislikes."
Mr Clegg enters tonight's second TV debate with a boost from the latest ComRes poll for The Independent and ITV News, which shows the Liberal Democrats have moved into second place. The Tories are on 35 per cent (unchanged), the Liberal Democrats on 27 per cent (up one point on the survey published yesterday) and Labour on 25 per cent (down one).
According to ComRes, 45 per cent of people believe Mr Clegg would make a good prime minister, compared to 44 per cent for David Cameron and 30 per cent for Gordon Brown. Some 57 per cent believe the Liberal Democrat leader could bring a fresh start for the country, with the Tory leader scoring 47 per cent and Mr Brown 19 per cent.
Mr Clegg outscores his two rivals on deserving a chance to run the country and understanding people's problems. But Mr Brown and Mr Cameron beat him when people were asked if they have a good idea of what the leaders stand for and of their party's key policies.
ComRes telephoned 1,015 GB adults between 19 and 20 April 2010. Data were weighted to be demographically representative of all GB adults. Data were also weighted by past vote recall. ComRes is a member of the British Polling Council and abides by its rules. Full details at comres.co.uk