Gordon Brown has said he has to "try harder" and "dig deeper" as party leaders return to the campaign trail after the final TV debate.
The PM, who snap polls suggested was seen to come third in the debate, said the TV clashes were over and decision time for voters was beginning.
Tory leader David Cameron said the election was "far from won".
Lib Dem leader Nick Clegg said the election was now a "two-horse race" between his party and the Tories.
In other election developments on Friday:
* The Conservatives send out a promises "contract" to voters in target seats
* Former PM Tony Blair says Labour has "every chance of succeeding" in the election on 6 May
* Nick Clegg says the election is now a two-horse race between the Lib Dems and Tories
* Alistair Darling says he is "not interested" in becoming the next Labour leader if Gordon Brown decides to stand down after the election
* Audience figures suggest 8.4m on average watched the last TV debate on the BBC
* The SNP and Plaid Cymru outline what they will do if there is a hung parliament
* Labour launches a poster campaign based around their child tax credit pledge - as a car crashes into a bus shelter nearby
* UKIP says it would axe a dozen quangos to save billions of pounds
* Thousands of postal votes are to be sent out again in the Vale of Glamorgan, a key marginal seat, after an error invalidated some forms.
Snap polls suggested Mr Cameron was seen to do best in the final live debate - watched by 8.4m people on average - peaking at 9.1m at 2100 BST.