THE Conservatives have failed to open up a clear lead in the election race because they are still perceived as the "nasty party," pollster Lord Ashcroft has claimed in a withering assessment of David Cameron's campaign.

The former Tory deputy chairman and donor said repeated attacks on Ed Miliband had reinforced a reputation Mr Cameron should have been striving hard to overturn.

Speaking at an event in Edinburgh organised by PR firm Charlotte Street Partners, he also rejected Labour and Lib Dem criticism of his constituency polling, which has shown large leads for the SNP in many once-safe seats.

Lord Ashcroft said the Conservatives should have been cruising to victory as Mr Cameron was seen by many as the best choice as prime minister and his party most capable of running the economy.

But he said: "The problem is by the last election the party had not overcome people's suspicions about it.

"The Tories needed to demonstrate in government that they really were on the side of ordinary people and could be trusted with public services like the NHS - but they now score no better on these measures than they did in 2010.

"Rather than being the party for people who want to get on, too many still think the Conservatives are just for people who have already made it."

He said the Tories had failed to eat into a solid base of Labour support, accounting for around 35 per cent of voters, because few of them had felt the benefits of the economic recovery.

He added: "Though Ed Miliband's approval ratings have risen over the campaign, Cameron retains a clear lead as the best available Prime Minister.

"But it seems to me that if one of your problems as a party is that some of the voters you need think you are "nasty," then launching personal attacks against your opponent is not the best way to capitalise on one of the few advantages you have.

"What they needed to do, and what I think they are very belatedly trying to begin to do, is to show voters why strong leadership makes a difference and why it should matter to them who occupies Numbers 10 and 11 Downing Street."

Lord Ashcroft has polled 27 Scottish seats this year, finding the SNP ahead in all but three.

The highest swing was 28.5 per cent from Labour to the SNP in Gordon Brown's old seat of Kirkcaldy & Cowdenbeath.

The pollster has predicted defeat for big names including Lib Dem Treasury Secretary Danny Alexander, former Lib Dem leader Charles Kennedy, Scots Labour leader Jim Murphy and shadow foreign secretary Douglas Alexander.

The Lib Dems and Labour have questioned the polling, arguing it fails to take personal recognition into account.

Referring to Mr Kennedy's battle, the pollster said: "I am very doubtful about this argument. It requires you to believe that, on the one hand, Charles's local reputation is so strong that he is unbeatable no matter what the national circumstances, and - at the same time - that even after 32 years as their MP nobody in his constituency will remember his name unless reminded by a pollster.

"In fact, as I found in my focus groups last week in Glasgow, Paisley and Edinburgh, people do think about their local MP when deciding how to vote, but it is just one of the considerations.

"For some it is the single most important factor, but for most others it is not.

"One of our Glasgow participants told us he had decided his very good Labour MP was going to have to 'suffer for the sins of others'."

He said Scottish Labour had been damaged by the perception it was a "branch office" of the UK party, by working alongside the Conservatives in last year's No campaign, and by appearing "indistinguishable" from the Tories in the eyes of many voters.