Nigel Farage is trailing in Thanet South, Nick Clegg is only three points ahead in Sheffield Hallam and Ed Miliband is leading in Doncaster North, according to a new batch of constituency polling undertaken by Lord Ashcroft.

Ashcroft has polled a total of 18 seats, including 11 held mainly by the Liberal Democrats with relatively large majorities and with the Conservatives in second place.

In Doncaster North Miliband holds a 29-point lead on 54%, with Ukip second on 25% and the Conservatives third on 13%, putting Labour well ahead of the Conservative and Ukip vote combined.

Nick Clegg is ahead of Labour by just three points in Sheffield Hallam, but if voters are not reminded of the Lib Dem candidate Labour is ahead on 28%, the Liberal Democrats are on 27% and the Conservatives on 2%. The Greens are on 10% and Ukip on 11%.

Clegg won 53% of the vote in 2010, so has clearly suffered a loss of support and will at least have to work for his seat in the final six month.

Nigel Farage is trailing in Thanet South, with Conservatives on 34%, Ukip on 29%, and Labour 26% and Liberal Democrats 7%.

One of the unknowns will be the degree to which Ashcroft’s constituency-by-constituency polling drives more tactical voting than normal, and the extent to which tactical voting operates in seats in which Ukip, Conservatives and Labour are all competitive.

Ashcroft found that in the Lib Dem-held Conservative targets there is an overall swing of only two points to the Tories – but only because the fall in the Lib Dem vote (13 points) was even bigger than that in the Conservative vote (9 points).

The Tories were ahead in North Devon, the seat held by Nick Harvey, where the Lib Dems are defending a majority of 5,821, and five points ahead in Portsmouth South, where the nine-point swing to the Tories suggests the controversial sitting Liberal Democrat MP, Mike Hancock, has damaged his party .

But the Liberal Democrat MPs who look like they will survive the coming carnage include John Hemming in Birmingham Yardley, Tom Brake in Carshalton and Wallington, Bob Russell in Colchester and Steve Webb in Thornbury and Yate.

Once voters are asked to think about their own constituency, Brake has a 20-point lead over his Tory rival, Webb a 23% lead and Russell a 14% lead. In Birmingham Yardley the Liberal Democrats are only 3% ahead. Norman Baker is ahead by 37 to 29 against the Tories in Lewes. Ed Davey, the energy secretary in Kingston and Surbiton, is ahead 38 to 30 and in Southport the Liberal Democrat John Pugh is ahead of the Tories by 37% to 24%. In Brecon and Radnorshire Roger Williams faces a battle to survive and has only a four-point lead over the Conservatives, with the Tories likely to make a strong appeal to the 17% planning to vote Ukip to swing back behind the Tories to drive the Liberal Democrats out.

In Wyre Forest the Conservatives are ahead and Ukip second with 27% of the vote, evidently benefiting from circumstances in which many local people have not voted for one of the main parties since 1997, but instead voted for a save the health service party.

The continuing damage to the Conservative vote remains, with 17% of 2010 Tory voters saying they would switch to Ukip in their own constituency – as would 13% of 2010 Labour voters and 10% of former Lib Dems.

But 92% of Conservative-Ukip defectors said they would rather have David Cameron as prime minister than Miliband, and two-thirds said their preferred outcome of the next election was a Conservative overall majority. These two findings will give encouragement to Cameron that he can drive down the Ukip vote by warning of the consequences of a Labour victory. In Doncaster North only 14% said they were satisfied with Cameron’s performance as prime minister; a further 24% said they were dissatisfied but preferred him to the alternative. Forty per cent of Miliband’s constituents said they would rather have him as prime minister than the incumbent.

Forty per cent of Miliband’s constituents said their preferred election outcome was a Labour government. But only 27% of them expected one.

• This article was amended on 2 December 2014. An earlier version used polling figures for Doncaster North published by Lord Ashcroft which he has now corrected. Ashcroft says the earlier figures included too many Conservative voters and not enough Labour voters, which made opinion in Doncaster North look considerably less favourable towards Ed Miliband than it actually is.

