The prime minister issued assurances that he understands concerns about Britain's place in the European Union and immigration.
Writing in The Sunday Telegraph, the Conservative leader says he understands voter scepticism about political promises but underlines his "clear and personal commitment" to quit the job if he cannot deliver his pledge to hold an in-out referendum.
In an apparent swipe at Ukip, he wrote: "To me, real patriotism isn’t running your country down at every opportunity, it is seeing its strengths and building on them — because time and again, the doom-mongers turn out to be wrong."
Cameron asked voters to embrace his 'optimism', as polls forecast the Tories will come third in the European elections
He added: "I have set out, very clearly, the key areas we are renegotiating on: keeping our border controls; cracking down on benefit tourism; securing more trade; getting more control over justice and home affairs; and getting Britain out of 'ever closer union'.
"Once we've negotiated, whatever the outcome, the British people will have the final say on our future in Europe, with that in-out referendum in 2017," he said.
"If you're thinking 'I've heard all this before' - I get it. I was watching too when Labour made Britain the door-mat of Europe and signed British taxpayers up to Eurozone bail-out funds. So I understand the scepticism. But if you want proof that we will deliver, just look at our record."
He adds: "I have made a clear and personal commitment: I would not be prime minister of a government unless we could carry out our pledge of an in-out referendum. This is a fundamental principle for me."
Polling has indicated Nigel Farage's party will come first in the UK's European Parliament results, with the Conservatives pushed into a humiliating third place.
In The Sunday Telegraph article, Cameron insists that only the Conservatives can deliver reform in the EU and a referendum.
The prime minister also takes an apparent swipe at Farage, mocking his pessimism, although he does not directly name him, with a reference to Dad's Army character Private Frazer.
He said: "There are politicians who refuse to be upbeat about Britain. To them, pessimism equals patriotism. They insist that our best days are behind us and that everything is bound to get worse. Like Private Frazer politicians they're perennially crying out that 'we're all doomed'."