Tory leader Michael Howard has accused Tony Blair of "lying to win elections" in his most personal attack to date.
Mr Howard called the government "wishy-washy"
He said Mr Blair had taken only one stand in the last eight years, over the Iraq war, "and he couldn't even tell the truth about that".
Mr Blair instead warned people they must vote if they valued Labour's achievements, especially in the "key marginals" which would decide the poll.
The Liberal Democrats have highlighted their policies on rural issues.
With the campaign past the half-way mark, three opinion polls in Sunday's newspapers suggest Labour leads the Conservatives by between 4% and 6%.
ICM/Sunday Telegraph puts Labour on 39%, Tories on 33%, Lib Dems on 23% and others on 8%.
A YouGov/Sunday Times poll has Labour on 37%, the Tories 33%, Lib Dems 23%, others 7%.
And Communicate Research/Independent on Sunday puts Labour on 40%, Tories 35%, Lib Dems 18% and others 7%.
Earlier, Chancellor Gordon Brown used a joint news conference with Mr Blair to appeal to disaffected Labour supporters considering switching to the Liberal Democrats to stick with the party.
Mr Brown said only a Labour government could bring about the sort of society traditionalist Labour voters believed in.
Mr Howard's attack came in a speech aimed at rallying the Conservative troops.
Tony Blair is focusing on turnout worries
He said people had a "duty" to take a stand at the election.
"How else do you think Mr Blair and his wishy-washy, pussyfooting government are going to get the message?" he asked.
Asked to respond to Mr Howard's comments, Mr Blair said the Tory leader could "conduct the campaign at that level if he wants to".
The prime minister argued the Tories had no programme to govern.
Labour's new campaign slogan says: "If you value it, vote for it", a message pushed by Mr Blair as he urged people to take account of a strong economy and public service investment.
Conservative foreign affairs spokesman Michael Ancram challenged Mr Blair to give a "clear account" of his part in the events leading up to the suicide of weapons expert Dr David Kelly in July 2003.
Dr Kelly was identified as the source of a BBC report that a dossier about Iraq's weapons of mass destruction capability was "sexed up". The government was subsequently cleared of the claim by the Hutton inquiry.
The Lib Dems have focused on rural affairs
The prime minister has previously maintained the government did not release Dr Kelly's name to the media but did confirm it when it was put to the MoD by journalists.
Earlier this week Mr Blair told the BBC's Jeremy Paxman: "I don't believe we had any option, however, but to disclose his name."
Mr Ancram has written to the Labour leader saying his latest comment "directly contradicts" what he had told reporters on 22 July 2003.
He wrote: "You were asked: 'Why did you authorise the naming of David Kelly?' You replied: 'That is completely untrue'.
"Again you were asked 'Did you authorise anyone in Downing Street or in the MoD to release David Kelly's name?' You answered: 'Emphatically not'.
"Questioned on why the government confirmed Kelly's identity, you said: 'That's a completely different matter once the name is out there'.
"But now you have admitted that you 'disclosed' the name.
"In light of the admission you have finally made, it would be right for you to come clean and explain in full your role in deciding how Dr Kelly's name was disclosed."
A Labour Party spokesman said Mr Blair had simply been restating the "well-known fact" that the MoD had confirmed Dr Kelly's name when it was put to the ministry by the Financial Times.
The Liberal Democrats are highlighting policies for sustainable communities with party president Simon Hughes pledging to help those people in the countryside who feared their way of life was being eroded.
Lib Dem leader Charles Kennedy meanwhile visited a farm in his Ross, Skye and Lochaber constituency where he predicted turnout on 5 May would be up compared with 2001.
He argued that voters would not want to see Mr Blair back in Number 10 with a big majority again "so he can do what he likes".
Elsewhere, the British National Party used St George's Day to launch its manifesto.