Thirteen sitting MPs have already announced recently that they will leave the Commons, but another 17 are expected to follow suit, the party chairman, Eric Pickles, told the Guardian.
The Tories still need to select candidates in about 100 constituencies. These include the 13 seats where Tory MPs have recently announced that they are standing down.
But Pickles said that he expected more MPs to quit and that "in total we might be looking at 30-ish Conservative-held seats [becoming vacant] before the general election". Pickles insisted that he expected Tory MPs to stand down not necessarily because they were afraid of losing, or had done anything wrong, but because they were demoralised.
"Some colleagues have found the process of expenses very wearing and they're very demotivated. They might not have had any problems themselves at all, but maybe they're feeling it's just time to move on," he said.
The Tories who have so far become casualties of the expenses furore include Andrew MacKay and his wife, Julie Kirkbride; Anthony Steen; Sir Peter Viggers – who notoriously spent £1,645 on a duck island; and Douglas Hogg. Nicholas and Ann Winterton were also criticised but said they were standing down because they had grown tired of the hectic life at
In May, David Cameron announced that he was reopening the list of those approved to apply for seats – the candidates' list – because he wanted to "refresh" the party with independent newcomers who did not have a conventional political background.
Pickles said the departures would give the party the chance to bring dozens of new-style Tories with little or no background in Conservative politics into parliament. He said that about 4,000 people had applied since then and up to 70% of them were "new people".
"We've got ourselves a good mix: lots of doctors, lots of social workers, lots of community nurses … people who've got involvement with the community outside the political process."
Pickles said that about 20 to 30 of these "new people" were likely to end up in parliament after the next election.
To help the new-style Tories get selected, the party is changing the way candidates are evaluated. In future, working in the community will be given equivalent weight to party political experience.
In his interview Pickles also said that he was worried about Gordon Brown standing down before the election and that winning "wouldn't be as simple" if Alan Johnson were leading Labour.
Pickles said that, although his party was well ahead in the polls, he was not convinced that an election victory was certain – not least because Brown could be replaced as Labour leader. "It's going to require a lot of graft from associations, a lot of graft from candidates and unremitting pressure between now and next May, because you can't rule out the possibility of something in the later part of the year or next spring," he said.
Asked what he meant, he said he was referring to Brown standing down. He said that it would be harder for the Tories to win with Johnson as prime minister, but he did not think replacing Brown would "fundamentally change the picture" because voters were angry with Labour, not just its leader.
Writing in today's News of the World, the business secretary, Lord Mandelson, insisted that Brown standing aside before the election, expected next spring, was "not an option".
He urged Labour MPs and activists not to "meekly hand over power" but to prepare for a concerted fightback.
Accepting that Labour is trailing badly in the polls and will enter the election campaign as the underdog, he asked: "Do we shrug our shoulders, accept our fate and meekly hand over power to an arrogant Conservative party which believes it can do nothing, sit back and be given power on a plate? Or do we fight back? … When we come back in the autumn the party needs to roll its sleeves up, pull together and concentrate."
