Michael Howard has claimed people have given up on Tony Blair "because they can see he's lost the plot".
Howard dismissed Labour's manifesto as "more of the same"
The Conservative Party leader warned voters that re-electing Mr Blair would mean more rises in taxes.
Mr Blair's programme for a third term includes a pledge not to raise income tax rates in the next Parliament.
But Mr Howard said: "All we have got is more of the same - more taxes, more talk... Mr Blair is not able to get a grip on the things that matter."
Charles Kennedy will launch the Lib Dem manifesto on Thursday after taking a short break following his son's birth.
On Wednesday, Mr Blair launched his party's nine-chapter manifesto, "Britain: Forward not back", with pledges on economic stability, health, education and tackling crime.
But Mr Howard said there was no use Mr Blair making any more promises after eight years in power.
"If Mr Blair gets in again, taxes will go up again," said Mr Howard.
"Everyone knows that his government has got the wrong priorities and the wrong values... he just talks and talks."
Mr Howard said people wanted more police on the streets. They also needed to know they would not come out of hospital more ill than when they went in.
He said voters had a choice on 5 May between a straightforward, accountable Conservative manifesto that had a grip on the issues that mattered, or rewarding the government "for eight years of broken promises".
While Tory plans would allow £4bn tax cuts in the party's first budget, Labour plans "have a big black hole in them", he said.
Mr Howard seized on a report from the International Monetary Fund (IMF) repeating a previous warning to Gordon Brown to rein in spending or face having to raise taxes.
"The IMF has dropped a tax bombshell on Labour," said Mr Howard during a visit to Nottingham.
He said the Tories did not need to promise not to raise income tax or National Insurance because they would cut taxes.
Earlier, Mr Howard denied claims his party was fuelling fears on immigration, saying they were talking "in a thoroughly responsible way" by promising a limit on the numbers entering Britain.
He also said voters had a right to know if Mr Blair had agreed to stand down early to make way for Chancellor Gordon Brown.
The Tories have also unveiled an advertisement to be played in 80 cinemas throughout the election campaign.
It features the song Take That Look Off Your Face while showing pictures of a smiling Tony Blair.
Mr Blair has said he is fighting his last election, and at his manifesto launch said he had already made it clear he would serve a full term if re-elected.
He said: "You can see this is a New Labour manifesto, on which everybody is agreed, and the stability of that New Labour message is there in my view now for the foreseeable future."
Mr Blair said National Insurance had been raised specifically to fund health service investment and insisted Labour could afford its 2005 manifesto commitments.
Lib Dem deputy leader Sir Menzies Campbell, setting out his party's health policies, said the election was not about Mr Blair personally.
"The presence or absence of Mr Blair in 18 months or two years doesn't seem to me to be relevant to this campaign.
"People are being asked to pass judgment on the government over which he has presided."