The contest between Texas Gov. George W. Bush and Arizona Sen. John McCain erupted today into an exchange of personal attacks, shattering the long-civil competition between the two leading candidates for the Republican presidential nomination.

Escalating their simmering dispute over taxes, McCain launched new radio and television ads in New Hampshire accusing Bush of "attack politics" and false charges to undermine the senator's tax cut plan. Bush responded that McCain had reverted to the use of "tired political tricks" to mask a flawed proposal and said McCain was guilty of misrepresenting Bush's tax plan.

The flare-up came barely two weeks after Bush and McCain publicly shook hands and vowed never to run attack ads in their nomination campaigns, signaling a fierce 10 days of campaigning ahead as the two candidates head for the crucial Feb. 1 New Hampshire primary.

What triggered the exchange was a Bush ad that was scheduled to run only a handful of times on New Hampshire television. Bush said in the ad that McCain's plan would force workers to pay taxes on $40 billion in fringe benefits they receive from their employers--a charge the McCain camp said the governor knows is false.

"I guess it was bound to happen," McCain declared in his ad today. "Now my opponent has started the political attacks after promising he wouldn't. Mr. Bush's attacks are wrong." McCain goes on to say "there is no tax increase" in his plan and adds that, unlike Bush, "I'm keeping my pledge" not to run negative ads.

Bush, campaigning in Iowa, denied he had aired a negative ad and said that McCain was trying "to turn the tables on people through old-style politics." Bush said he and McCain have "an honest disagreement" over the definition of employee fringe benefits but that McCain has resorted to "name-calling" rather than resolving the dispute.

The Texas governor objected to an earlier McCain ad that he said claims that he "doesn't save a dime for Social Security. I save 20 trillion dimes for Social Security." Bush was referring to his plan to ensure that the estimated $2 trillion in Social Security revenue that will build up over the next decade is not spent on other programs.

At a news conference in Greenville, S.C., where he began a two-day swing in the state that will hold the first southern primary Feb. 19, McCain was asked whether the New Hampshire commercial is an "attack ad" of the type he has vowed not to use. "Of course not," McCain said. "It is a direct response to an ad he's been running saying that I'm trying to raise taxes by $40 billion."

McCain spokesman Howard Opinsky insisted this was no mere policy disagreement. "We had to respond to what Governor Bush knows are false accusations," he said. "Airing an ad he knows to be false is a broken promise on his part."

Amid reports that the Bush campaign had stopped broadcasting the ad on fringe benefits, McCain told reporters that if Bush stopped using his ad he would do the same. "If he'll take down his ad our ad will come down just as soon as we can," he said. "But I have to respond. . . . In American politics, it is incumbent on you to respond when there is paid advertising that is totally false on an important issue."

Mike Murphy, a McCain strategist, said that the earliest the McCain ad could be pulled off the air was Monday. He said the offer to drop the spot was contingent on the Bush campaign not replacing its tax ad with another commercial that the McCain campaign considers negative.

The contretemps reflects an increasingly common political tactic of responding to substantive criticism by accusing an opponent of unfair negative campaigning. Ed Gillespie, a GOP strategist not aligned with any campaign, said that for McCain "to dub that an attack ad is like giving 'Bambi' an X rating because the deer are naked in the forest scene."

Today's eruption had been building since last Saturday's Republican debate here in Des Moines. Bush questioned whether McCain's plan would tax workers for such fringe benefits as education expenses, parking and transit assistance, as his written proposal had suggested.

McCain was slow to clarify exactly what the provision in his plan meant, but earlier this week issued a memo indicating that his plan would deny companies the right to deduct the costs of parking, free meals or transportation provided to employees.

The campaign also issued an analysis from an Arlington economic consulting firm maintaining that the provision, as clarified, would raise $4 billion over five years, not the $40 billion as charged by Bush.

Bush advisers continued to dismiss those clarifications today, even though Bush said Thursday "I take him for his word" that McCain had clarified the impact of the disputed provision.

Bush spokesman Ari Fleischer defended the Bush ad as an accurate portrayal of McCain's tax package as it was unveiled 10 days ago. "Our ad is an accurate portrayal of his written document," Fleischer said. "They continue to attack us for what is a fair and accurate portrayal of their plan."

Bush officials also said the revisions in the McCain plan raise new questions about how McCain would pay for his proposed tax cuts, which would cost about $240 billion over five years.

Opinsky accused the Bush camp of a continuing strategy of deliberate misrepresentation. "If our words . . . not as precise as needed [in the original plan], that's a failure on our part," he said. "But it should have been enough to be very clear to them that we were not talking about employee benefits. The fact that we protested should have been their first indication."

Kurtz reported from Washington; staff writer Edward Walsh in Greenville, S.C., contributed to this report.