WASHINGTON — Senator McCain, whose tensions with social conservatives have become a drag on his presidential campaign, is also at war with economic conservatives who favor tax cuts.

Mr. McCain’s votes against President Bush’s tax cuts in 2001 and 2003 infuriated them. He is being pressed to explain why be broke with an element of the Bush agenda that most of them consider an unqualified success.

In one exchange last month, Mr. McCain criticized a prominent anti-tax group called the Club for Growth, saying he wasn’t sure what he and the group had in common. In response, the group demanded that Mr. McCain apologize for his tax votes.

Mr. McCain’s leading rivals have taken advantage of the tensions. Mitt Romney, the former Massachusetts governor, started airing a television ad in Iowa and New Hampshire last week that cast him as a budget hawk.

“If I’m elected president, I’m going to cap nondefense discretionary spending at inflation minus 1%. … And if Congress sends me a budget that exceeds that cap, I will veto that budget,” he says in the ad.

Mr. Romney says he would cut capital gains taxes for middle-class investors, and aides say he would trim marginal and corporate tax rates.

Mayor Giuliani says he cut taxes 23 times in the mostly Democratic city and turned a budget deficit into a surplus. Mr. Giuliani said he liked the idea of a flat tax, but it was not feasible.

Conservatives say Mr. McCain’s problems are also driven by his refusal — alone among leading GOP candidates — to call for eliminating the estate tax.

Mr. McCain says he would retain the estate tax but cut the top rate from 60% to about 35%. His advisers say he is “deeply interested in cutting taxes” to make them “fairer, flatter, simpler,” but he has not announced a cut in marginal income tax rates.