Gordon Brown is hoping for a "progressive alliance" of Labour and Liberal Democrat supporters to work together to keep the Conservatives out of power.
The Prime Minister makes the comment in an interview with The Independent newspaper.
He also says that he wants a "new politics", with the two parties creating an informal alliance around common concerns, but acknowledging differences.
Meanwhile, Tory leader David Cameron has claimed that a balanced parliament would be bad for the country, and has suggested that it would be irresponsible for people to vote for anything other than a majority for one party - which he hopes will be his.
Commentators say that Mr Brown's plea echoes past Labour promises about "new politics", political reform and alliances - but critics point out that these have dematerialised when electoral fortunes change.
Mr Brown's pitch is that: "If you want a referendum on new politics, you've got to consider voting Labour. We are the only party committed to a referendum on it. You won't get one with the Tories."
Other parties, including the Lib Dems, SNP, Plaid and the Greens, point out that they are promising reform rather than an unspecified referendum on it.
Mr Brown went on to say that a Conservative victory would mean "a change of personnel and a return to the old politics", whereas Labour was now "serious" about revamping the UK's electoral system.
However, in an interview with the Daily Telegraph, Liberal Democrat leader Nick Clegg says Mr Brown is now "a desperate politician".
The Liberal Democrat leader has indicated that he might find it difficult to do a deal with Brown in the event of a hung Parliament, but he has said the same about the Tories too, which critics point out would be an unsustainable position.
"Do I think Labour delivered fairness? No. Do I think the Labour Party, in its heart, has a faith in civil liberties? No. They are clutching at straws," said Mr Clegg.
The three leaders of the big, corporate parties are trying to re-position themselves in the public view ahead of the second leaders' TV debate tomorrow.
Mr Clegg has been criticised for attacking the "old parties" while behaving in the same way as them in dismissing others.
Mr Brown tried to align himself with Clegg against the Conservative leader, David Cameron, in the first debate, saying repeatedly "I agree with Nick".
Campaigners for a balanced or 'hung' parliament (http://hang-em.com/) say that the only way to pressure the big party leaders into change is to put them in a position where they have to deal rather than posture.
A series of opinion polls published in today's newspapers gave a mixed picture of the state of the three big parties, reports ePolitix.com.
The YouGov daily tracker for the Sun suggested that the Liberal Democrats were up three points from 20 April, on 34 per cent, the Conservatives down two to 31 per cent and Labour down one to 26 per cent.
The weekly Populus poll for The Times puts the Lib Dems on 31 per cent, a 10 point increase on their last poll a week ago, one point behind the Tories who are down four on 32 per cent, while Labour is down five on 28 per cent.
Ekklesia is monitoring the polls regularly, particularly examining the smaller parties and 'other' categories which the 'big three' and media are tending to by-pass. See: http://www.ekklesia.co.uk/tags/7671