Prime Minister Brown said only Labour could take the country "forwards" and would "always be on your side".
Tory leader Mr Cameron said a victory for his party would bring "hope, optimism and change".
Urging voters to trust their "better instincts", Lib Dem leader Nick Clegg said this election is "our chance".
ICM poll for The Guardian puts the Tories up three points on 36%, Labour unchanged on 28% and the Liberal Democrats down two points on 26%.