"We've had a positive response to a positive campaign about safeguarding our economy," said Cameron, as if he had always expected to win so emphatically.
The truth was different.
Before it became clear he had won, some in his center-right Conservative Party feared he had run a dull campaign that failed to shift apparently tied opinion polls.
Others in the party, famous for ruthlessly junking predecessors such as triple election-winner Margaret Thatcher, thought his days were numbered even if he won because he was unlikely to win big.
He forgot the name of his football team at one point, was accused of dodging TV debates, and had sometimes struggled to hold his party together.
Seeking to lift his game, a gesticulating and shirt-sleeved Cameron vehemently described himself as "pumped up" at one campaign appearance widely derided by critics. But that had to be set against Labour leader Ed Miliband's much-ridiculed efforts to convince voters that "Hell yes, I'm tough enough".
Cameron, guided by his Australian campaign adviser Lynton Crosby, spent six weeks hammering home just two messages: Vote Conservative to secure economic recovery, and stop Labour coming to power backed by Scottish nationalists.
Crosby's strategy was that "you can't fatten a pig on market day". That meant voters were bombarded with a message in the hope that relentless repetition would help it "take".
"The Lynton Crosby strategy came through in the end," one Conservative activist in Cameron's Oxfordshire constituency, who declined to be named, told Reuters.
As he addressed supporters on Friday, Cameron savored proving his doubters wrong.
"The pundits got it wrong, the pollsters got it wrong, the commentators got it wrong," he said. "This is the sweetest victory of them all."
Conservative staffers said they were surprised by the scale of their victory.
Many put it down to English horror at the prospect of Scottish nationalists wielding influence over swathes of the United Kingdom which they still want to leave despite losing an independence referendum last year.
"It's got to be the Scottish National Party angle," one jubilant Conservative activist who declined to be named said. "More than any line in any election, that one has really cut through to people we meet on the doorstep."
The SNP didn't run on an independence ticket this time, drawing in voters who want to stay in the United Kingdom but want a stronger Scottish voice in British politics.
It was a strategy that won them a landslide, securing 56 of Scotland's 59 parliamentary seats.
It repeatedly offered to help Labour come to power "to lock out Cameron". Miliband ruled out deals with the SNP, but failed to dispel voters' doubts he would relent and make a pact with the nationalists.
For many in England that was a reason not to vote Labour.
Though it didn't initially appear to have the impact he had hoped for, Cameron's economic record gave him a lead over Miliband on economic competence.
The fact that real wage growth only picked up in the months before the election caused jitters in the Cameron camp. But he was able to deliver record low inflation, high employment and cheap mortgages.
And crucially, he told Britons they would feel the benefits of the recovery if they gave him another five years.
"This somehow actually had more traction (than people thought)," said Grants Shapps, Conservative party chairman.
Cameron's pledges to cut welfare spending sharply angered Labour supporters. But they went down well with many voters who resented claimants regularly portrayed as feckless parasites.
But perhaps Cameron's best asset was Miliband, nicknamed "Red Ed" by his detractors.
He began the campaign cast by right-leaning newspapers as a socially awkward geek with neither gravitas nor policies. His party had left Britain with its biggest peacetime deficit when it left office in 2010.
Miliband tried to repair Labour's battered reputation for fiscal responsibility but refused to say it had borrowed too much, angering some voters. He forgot key passages of a speech on the economy and immigration at Labour's last conference before the election.
And in a move that dismayed some supporters, he commissioned a stone tablet engraved with his election promises which critics ironically compared to Moses' Ten Commandments.
During the campaign, Miliband was perceived to have outperformed low expectations and to have improved his ratings.
But it wasn't enough.
"His ratings improved but they are still much below David Cameron in terms of competence," said Ben Page, chief executive of pollster Ipsos MORI.
Perhaps most importantly, Miliband's big gamble didn't come off. One of his predecessors, Tony Blair, had led Labour to three election victories by anchoring the party in the center ground. But Miliband shifted to the left, promising to raise taxes and spending and to intervene in markets to right what he perceived as unfair imbalances.
"We failed to offer a compelling vision of the future," said Tristram Hunt, Labour's education spokesman.
Some blamed David Axelrod, the former Obama adviser, who helped coordinate Labour's campaign.
"To a certain extent he didn't succeed in creating a campaign that got to everybody across the country and that's what you're going to need to do if you're going to get into government again," said Jacqui Smith, a former Labour minister.
COALITION PARTNER WOES
Cameron was also boosted by a collapse in support for his coalition partner, the Liberal Democrats.
Opinion polls had suggested the centrist party, with whom Cameron had governed since 2010, had paid a heavy price for going into government with him.
Many supporters felt it had betrayed its principles by going into coalition with Cameron and could not forgive it for what they saw as a U-turn on student tuition fees.
It was expected to do badly, but not even its fiercest critics predicted it would win just eight seats, down from 57 in 2010.
The Conservatives won 27 seats from the Liberal Democrats, claiming the scalps of two of their senior cabinet ministers, Business Secretary Vince Cable and Energy Secretary Ed Davey.
Equally, a potential threat to Cameron from the anti-EU UK Independence Party never really materialized.
It had threatened to split the Conservative vote and it did win millions of votes, but Britain's first-past-the-post electoral system meant it won only one seat in the end.
But it was Labour's collapse in Scotland, a traditional stronghold, that lost Miliband the most seats. In 2010, Labour won 41 seats there. This time it won just one.