Ed Miliband made an audacious bid to outflank David Cameron on the economy by calling for the 10p rate of income tax to be brought back.
The Labour leader insisted reintroducing the band - controversially scrapped by Gordon Brown - would make society fairer.
In a keynote speech, he said the move could be funded by a new "mansion tax" on homes worth more than £2 million.
The announcement was a surprise inclusion in the address, which had been billed as featuring no significant policy - a prospect mocked by the Prime Minister in the Commons.
Tory backbenchers have been campaigning for the 10p rate to be brought back in next month's Budget, and taking questions in the House Mr Cameron appeared to hint he was open to the idea.
Speaking in Bedford - where Tory premier Harold Macmillan delivered his famous "You've never had it so good" line in 1957 - Mr Miliband said: "A One Nation Labour Budget next month would lay the foundations for a recovery made by the many, not just a few at the top.
"Let me tell you about one crucial choice we would make, which is different from this Government. We would tax houses worth over £2 million. And we would use the money to cut taxes for working people.
"We would put right a mistake made by Gordon Brown and the last Labour government.
"We would use the money raised by a mansion tax to reintroduce a lower 10p starting rate of tax, with the size of the band depending on the amount raised.
"This would benefit 25 million basic rate taxpayers - moving Labour on from the past and putting Labour where it should always have been - on the side of working people."
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