Mitt Romney, who is one of the wealthiest men ever to seek the presidency, said on Wednesday that he had no intention of releasing his tax returns if he became the Republican presidential nominee, breaking with a long tradition in both parties.

Mr. Romney made the statement in an interview with MSNBC on Wednesday, but the network did not show that part of the interview. Mr. Romney, a multimillionaire who made his fortune running a private equity firm, was asked whether he planned to release his tax return.

“I doubt it,” Mr. Romney said, according to a transcript of the interview provided by NBC News. “I will provide all the financial info, which is an extraordinary pile of documents which show investments and so forth.”

“But you won’t do the tax returns?” asked Chuck Todd, host of “The Daily Rundown.”

“I don’t intend to release the tax returns. I don’t,” Mr. Romney responded.

A spokesman for President Obama‘s re-election campaign blasted Mr. Romney and questioned whether he had something to hide in his finances.

“Why does Governor Romney feel like he can play by a different set of rules?” said Ben LaBolt, a spokesman for the Obama campaign. “What is it that he doesn’t want the American people to see? Governor Romney, who has favored secrecy over openness time after time, should live up to the same standard of disclosure his father and others set.”

The Democratic National Committee took only hours on Thursday to unveil a new Web site, www.whatmittpays.com. The site allows visitors to select their income level and compare how much less they would have paid if they were taxed at what the group assumes would be Mr. Romney’s lower rate.

Presidential candidates from both parties — including John McCain, Barack Obama, George W. Bush, Al Gore and even George Romney, Mr. Romney’s father, who ran in 1968 — have routinely released their tax returns.

Mr. Romney’s personal wealth — said to be between $190 million and $250 million — helped to fuel his first campaign for the presidency in 2008, when he contributed more than $40 million of his own money.

A story in The New York Times this week reported that Mr. Romney continues to increase his wealth through a retirement agreement with Bain Capital, the private equity firm he founded, which still pays him a share of profits long after his departure.

The arrangement with Bain also most likely means that much of Mr. Romney’s income is taxed at a much lower rate than ordinary income under a provision aimed at hedge fund and private equity managers. Releasing his tax returns would allow Mr. Romney’s rivals to calculate the effective tax rate he is paying.

That could be politically difficult for Mr. Romney, who often talks about the need to help middle-class Americans get through the current economic difficulties.

Throughout his political career, Mr. Romney has refused to release his personal tax returns. During his 1994 Senate campaign in Massachusetts, Mr. Romney challenged the incumbent, Edward M. Kennedy, a Democrat, to release his tax returns — though Mr. Romney did not release his own.

Mr. Todd asked Mr. Romney about that during the interview on Wednesday.

“I never say never,” Mr. Romney said, but added, “I don’t intend to do so.”