The surge in support for Ukip has split the Tory vote, leaving the path to Downing Street open for Ed Miliband despite his persistently low poll ratings, according to figures from key marginal seats.
Nigel Farage's party has seen its poll ratings in Tory marginals rise threefold since the last general election, according to polling data from Lord Ashcroft, the billionaire former Tory party deputy chairman.
Farage's gains could translate into the loss of 40 key Tory marginals to Labour at the next general election, with Labour widening the gap in the seats that represent its biggest targets for 2015.
In the 40 Tory seats with the tightest majorities, Labour increased its lead from nine to 14 points over the last two years, largely because of the defection of Tory voters to Ukip.
Labour's ratings in those seats fell one percentage point to 43% in the last two years. However, support for the Conservatives fell to 29% over the same period, with the Lib Dems on 8%.
Ashcroft polled nearly 13,000 voters in the 40 Conservative seats with the smallest majorities, 32 of which the party is defending against Labour, and eight where the Liberal Democrats came second in 2010.
Overall, the figures showed an 8.5% swing away from the Tories in the 32 seats the party is defending against Labour. At a general election, the results would bring Labour all 32 of those seats, along with a further 66 if the swing was repeated in Tory-Labour contests elsewhere.
"The tripling of Ukip's vote share in marginal seats since 2010 threatens to put Ed Miliband in Downing Street in spite of Labour's lukewarm appeal," Ashcroft said.
"For most of the summer Labour's poll lead has been in single digits. Though enough for victory at a general election, this is hardly a comfortable margin for Ed Miliband at this stage of the parliament - particularly since, as I have found in my previous research, Labour's support is far from firm."
However, Ashcroft said there were grounds for the Tories not to feel too despondent.
"Voters [in Tory marginal] are slightly more likely than not to think Britain is heading in the right direction, and the opposition has not fully won their confidence," he said.
"The Conservatives are more likely to be seen as willing to take tough decisions, and being clear about what they stand for."
The Tories however look slightly less vulnerable in the seats the party is defending against their Lib Dem coalition partners, according to Ashcroft's data,
Here, the Tory share was down 9 points since the last election, with Ukip emerging as the main beneficiary.
However, support for the the Lib Dems fell even further, down 10 points, as former supporters deserted the party defecting to Labour.
More cheering for Ashcroft was data that showed voters still saw David Cameron as a better leader than Ed Miliband, with 38% saying Cameron would make the best prime minister, compared to 28% for the Labour leader.
Aschcroft's findings came as a YouGov poll for the Sunday Times showed Labour five points clear on 38%, the Conservatives on 33%, and Ukip on 12%, putting the the Lib Dems into fourth place on 9%.