Cameron cannot win a parliamentary majority in a national election due in 2015 without support from traditional right-leaning supporters who polls show are ditching his Conservatives in favour of the UK Independence Party (UKIP), commentators say.
A week after UKIP notched up its biggest share of the vote in an election for a parliamentary seat, a YouGov poll found 36% of respondents would potentially back them in a national poll.
UKIP, which has no lawmakers in the Westminster parliament, has siphoned off support from the Conservative Party by denouncing EU bureaucracy and immigration from eastern Europe, and tapping into what its leader Nigel Farage says is a widespread feeling that voter concerns have been ignored.
Last Thursday, it took second place in a closely watched election for a vacant parliamentary seat in southern England, beating the Conservatives into third, recording its best result in a so-called by-election and echoing the pattern of other recent votes.
That raised questions about Cameron's leadership and whether he can win a parliamentary majority in 2015 after five years of shared power with the centre-left Liberal Democrats.
Cameron has already promised an in-out referendum by the end of 2017 on Britain's membership of the EU, but last week's result and the YouGov poll suggest that he has failed to woo sceptical voters deserting him for UKIP.
"Sixty percent of UKIP's current supporters voted Conservative in 2010 - a clear majority," said Peter Kellner, president of polling company YouGov, in a blog post.
However, the Conservatives might be cheered to note that only 4% of those surveyed by YouGov said they would definitely vote for UKIP in a national election, suggesting they could win back disillusioned voters.
Overall, the poll put the opposition Labour Party on 40% and the Conservatives on 31%, with the Liberal Democrats, tying third with UKIP on 12 percent.
Perhaps surprisingly, YouGov said UKIP was also drawing support from voters who previously backed Deputy Prime Minister Nick Clegg's pro-EU Liberal Democrats, benefiting from a backlash against the mainstream parties.
"This demonstrates that we have appeal across all political boundaries because we talk about things that matter to ordinary people, not just the cosseted political class," UKIP leader Nigel Farage, a member of the European Parliament, said of the survey findings.