Ed Miliband is failing to repeat Tony Blair's success in winning over former Tory voters and will have to rely on people returning to Labour as well as ex-Lib Dem converts to win a majority, a new polling analysis suggests.

Research by the New Fabian Society finds just 400,000 voters have moved from Conservatives to Labour since the last election which, if unchanged on polling day, would mean Labour had made only tiny inroads into Tory heartlands. The Fabians' investigations – after a week in which proposed boundary changes, which would have benefited the Tories by around 20 seats in 2015, were defeated – will alarm those in the party who believe Labour is still not doing enough to attract potential Conservative defectors. The study makes clear that, if apathy were to set in among 2010 non-voters who say they intend to back Labour again, Miliband will struggle to win an overall majority.

Blair set out from the start of his leadership to woo disaffected Tories with his "New Labour" branding and constant message that being pro-business complemented, rather than conflicted with, Labour values. The result was a mass movement of Tories to Labour and a landslide victory.

Miliband, who won the Labour leadership with a large chunk of union votes, is seen by some Blairites in his party as pitching his case more to those on the left than Blair. The Tories have targeted him as "Red Ed" and in the pocket of the unions.

The research was based on months of detailed analysis of a YouGov poll that charted new ground by looking at the voting intentions of people who did not vote in 2010. It found that an estimated 1.4 million people who did not vote at the last election now say that they intend to vote Labour. This has helped increase Labour's rating in the polls by around 5% and could represent around another 40 seats.

Many of these 2010 "no-showers", the study says, are likely to be former Labour voters who became disillusioned with the party in its latter years in office. It also deduces that Labour has won over some 2.3 million voters who chose the Lib Dems in 2010. If current levels of ex-Labour support and the Lib Dem deserters do not drift away or decide not to vote, Labour would have enough to put Miliband in Downing Street.

Andrew Harrop, the Fabians' general secretary and author of the study, says that, with few Tories showing signs of moving directly to Labour, the key for Miliband could be whether he can motivate 2010 Labour "no-showers" to turn up on polling day. Harrop calls for a huge effort by Labour to invigorate grassroots voters similar to the Obama campaign that secured victory for the Democrats in the US last year. "Ed Miliband is not Tony Blair and he'll need to win power in his own way. Blair's success was based on winning over disillusioned ex-Tories who are so far resisting Miliband's appeal," said Harrop.

"Instead Ed has won the backing of people who had given up on voting as well as former Lib Dems. The Fabian research shows that together there are enough of them for Labour to win a majority. The challenge for Labour is to turn this mid-term support into votes in 2015. This will take a huge organisational effort, with strong parallels to the Obama campaign's efforts to mobilise sympathetic voters."