WASHINGTON — More than two years after losing his bid for the White House, Senator Kerry, a Democrat of Massachusetts, exacted a measure of revenge against his political foes yesterday by helping derail the diplomatic nomination of a Republican fundraiser.

President Bush withdrew the nomination of St. Louis businessman Sam Fox to be ambassador to Belgium after Democrats denounced Mr. Fox for his 2004 donation to the Swift Boat Veterans for Truth.

The group’s TV ads, which claimed that Mr. Kerry exaggerated his military record in Vietnam, were viewed as a major factor in Mr. Kerry losing the election.

Mr. Bush’s action was announced quietly minutes before the Senate Foreign Relations Committee was to have voted on the nomination.

“His nomination would not have passed today if the vote had been called up,” said White House spokeswoman Dana Perino.

During a confirmation hearing last month, Mr. Kerry grilled Mr. Fox about why he had given $50,000 to a group that was “smearing and spreading lies” about him. Mr. Kerry seemed to be seeking an apology but Mr. Fox didn’t budge, saying he simply gave when asked.

“Sam Fox had every opportunity to disavow the politics of personal destruction and to embrace the truth,” Mr. Kerry said yesterday. “He chose not to. The White House made the right decision to withdraw the nomination. I hope this signals a new day in political discourse.”

Mr. Fox, 77, is national chairman of the Jewish Republican Coalition and was deemed a “ranger” by Mr. Bush’s 2004 campaign for raising at least $200,000.

He is founder and chairman of the Clayton, Mo.-based Harbour Group, which specializes in the takeover of manufacturing companies.

Mr. Fox has donated millions of dollars to Republican candidates and causes since the 1990s.

Big-money contributors are often rewarded with ambassador posts.

Contributions to political groups are rarely considered grounds for protest for fear there would be a tit-for-tat when the White House changes parties.

But the chorus of opposition to Mr. Fox grew louder a day before the vote was set, when Mr. Kerry’s Vietnam crew mates sent a letter Tuesday urging committee members to oppose Mr. Fox’s nomination.

“In our judgment, those who finance smears and lies of combat veterans don’t deserve to represent America on the world stage,” said the letter signed by 11 Vietnam Swift Boat veterans who served with Mr. Kerry.

Fueling the political undertones was the presence of three Democratic presidential hopefuls on the committee — the chairman, Senator Biden, a Democrat of Delaware; Senator Obama, a Democrat of Illinois, and Senator Dodd, a Democrat of Connecticut.

“I had serious concerns about Mr. Fox’s candor, judgment, and qualifications for this important post, and I am pleased that the Bush administration acknowledged that it would not be able to muster the votes to confirm his nomination,” Mr. Obama said.

Even after the February 27 confirmation hearing, Mr. Kerry gave Mr. Fox an opportunity to show contrition in responses to several written follow-up questions.

Mr. Fox insisted he did not know how his money would be spent or exactly what message the Swift Boats were pushing.

“I did it because politically it’s necessary if the other side’s doing it,” Mr. Fox told Mr. Kerry.