"This is a real career-defining...Country-defining election that we face now in less than a week's time," the Conservative leader said as he urged people to vote on May 7.
Opposition Labour leader Ed Miliband immediately seized on the apparent slip, accusing Cameron of being more focused on his legacy than the state of the country.
"That tells you the difference between us. He thinks this election is about him. I think it's about you the British people," he told voters on the campaign trail.
It is not Cameron's first election gaffe -- last week he got the name of his favourite football team wrong, which commentators put down to campaign exhaustion.
With less than a week to go, the Conservatives and Labour are neck and neck in the opinion polls, with neither on course to win the parliamentary majority that would enable them to govern alone.