With the opinion polls still suggesting the Tories will fall short of an overall majority after polling today, he warned that the “worst thing” would be another five years of Gordon Brown.
On a final day of frenetic campaigning by the three party leaders, Brown hammered home his message that only Labour could guarantee the economic recovery, declaring: “This is not a Conservative moment.”
Meanwhile, Liberal Democrat leader Nick Clegg urged his supporters to hold firm in the face of pressure from the two larger parties.
After an exhausting month for all three leaders, criss-crossing the country, Cameron used one of his final campaign visits – to a school in Newtown in Powys – to spur on his supporters to make one last push.
“Let us raise our sights and imagine a better, stronger country with a Conservative government from Friday,” he said.
“We have just a few hours left, but we have got to make those hours count because the worst thing we could have is another five years of Gordon Brown.”
Cameron, who earlier admitted that it was a “nerve-wracking time”, accused Labour of running a “disgraceful and negative” campaign about Tory spending cuts.
“If we get to lead this country, we will take everyone with us – the elderly, the frail, the vulnerable, the poorest,” he said.
Brown, in a speech to students at Bradford University, said Conservative plans for “the wrong cuts in the wrong place at the wrong time” would put the recovery at risk.
“I ask you when you go in to the polling booths to ask yourself: who will stand up for you? Who, when the forces of privilege raise their voice, will raise their voice for you?” he said.
“And who will fight for your family because they know what it is to walk in your shoes? And I say with humility: I will.”
Meanwhile, Clegg, campaigning in Eastbourne, sought to shore up his vote in the face of Labour attacks.
“Use your vote, make your voice heard,” he told his supporters.
“Just imagine how you are going to feel if you wake up on Friday morning and discover instead that the Labour Party and Gordon Brown are back in power, having let you down.
“Just imagine how you are going to feel if you wake up on Friday morning and you find the Conservatives and David Cameron in No 10, just because they think they are entitled to have a turn.”
Earlier, Home Secretary Alan Johnson launched a withering personal attack on the Lib Dem leader, branding him “arrogant” and suggesting that his earlier surge in support had finally run out of steam.
“The Liberal Democrats are on a slow puncture and the air is coming out of the tyre,” he said. “What people see is a sort of Euro-friendly party that would have us in the single currency. They would have an amnesty for illegal immigrants, they would allow asylum seekers to work, which is utter, utter madness.”
He said that Mr Clegg’s style had become “a bit grating” and he denounced what he said were the Lib Dem leader’s pretensions to dictate who would be prime minister in the event of a hung parliament.
“It is arrogant of Nick Clegg to decide who he is going to pick and choose, thinking that he is the kingmaker,” he said.
His onslaught came as two overnight opinion polls put the Lib Dems back in third place, suggesting that Labour strategists believe they are vulnerable to a classic third party “squeeze” in the hours before polling.
After mixed signals from Labour over whether its supporters should vote tactically to keep the Tories out, Brown made clear he wanted to maximise Labour’s share of the popular vote to strengthen his hand if there is a hung parliament.
“I want every Labour voter to vote Labour,” he said. “People will judge us on the number of votes we have got as well as the number of seats.”
Following shadow business secretary Ken Clarke warning on Tuesday that the chances of an outright Conservative majority were “slim”, shadow children’s secretary Michael Gove insisted that they still hoped to avoid a hung parliament.
“The only way that we can get rid of the deals and the sort of backroom nonsense is if we have a clear mandate for a new government.
“The only way we can guarantee change is by having an overall majority for the Conservatives,” he said.