As polls continue to point to a hung parliament, with the Tories' lead still short of what David Cameron needs to be sure of a Commons majority, the leader of the opposition rallied activists to use the "40 days and 40 nights" before polling day to convince voters to ditch Labour.
"Imagine the brass neck of what he's saying – 'vote for me, I doubled the national debt; vote for me, I put up taxes 178 times; vote for me, I let 80,000 criminals out of prison early; vote for me, I sent troops into battle while cutting the defence budget and not telling the truth about it till the Iraq Inquiry'," the Conservative party leader said in a speech in Milton Keynes.
"It's the biggest display of brass neck in British history and it's this party that's got to make sure we put a stop to it."
Mr Cameron admitted the election would be a "close fight" but insisted it was one the Conservatives had to win for the good of the country.
And yesterday, he accused the Prime Minister of being "in hock" to the unions.
Amid industrial unrest at BA and on the railways, the Tory leader said Mr Brown had shown "a certain weakness".
"I think we should expect our Prime Minister to be pretty clear about two things," he said in a pre-recorded interview.
"One is there are businesses like the railways, like British Airways that obviously need to modernise and to restructure, to keep up with their competitors and with the competition.
"And secondly if people want to go to work they should be supported to go to work rather than have a Prime Minister who sits on the fence, I would argue, partly because he's so in hock to the unions, he's so reliant on them for all of the money and everything else that goes with it."
But Mr Brown said his Tory counterpart was "totally wrong".
"The number of days lost in industrial disputes during the period of our Labour government has been a tenth of what it was under the Conservative government," he said.
"But we have been very tough about this British Airways strike, we have said it is not in the public interest, we have said it is not in British Airways' interest and we have said we don't think it is in the workers' interests."
The Prime Minister earlier acknowledged Labour was "the underdog", but insisted the party would "never give up".
"We are fighting for Britain's future – and we intend to win," he said as he set out Labour's five key pledges for the election, which he insisted were "substantial, deliverable and carefully costed".
They were to secure the recovery, raise family living standards, build a "hi-tech" economy, protect front-line investment in policing, schools, childcare and the NHS, and strengthen fairness in communities.
Mr Brown said voters faced a choice at the forthcoming election as historic as that at the 1997 election.
"In the history of each nation there are moments of clear decision, moments when paths are chosen and decisions made that impact not only the months or years to come, but shape the whole course of the decades that follow," he said. "So it was in 1997, and so it is today, in 2010."
Polls released on the weekend suggested the Budget, the last before the election, had made little difference to the Tories' prospects. The Conservatives' lead was between five and eight points, according to three different polls, none of which is enough to install Mr Cameron in Number 10.
A majority of the public, some 51 per cent, feels Alistair Darling's financial statement last Wednesday was bad, a YouGov survey found.
An ICM poll for the News of the World suggested 24 per cent of people were less likely to vote for Labour, compared with 9 per cent who were more likely.
But the polls gave conflicting indications of the public mood.
The YouGov poll suggested Labour had narrowed the gap from seven to five points since last week. It put the Tories down one on 37 per cent, Labour up one on 32 per cent, and the Liberal Democrats unchanged on 19 per cent.