Yesterday, Howard Dean and Ralph Nader, brothers by temperament and opponents by choice, met across a platform and grappled with disagreements that go to the core of the presidential election.

Dean attacked Nader for taking campaign contributions from Republicans and sullying his integrity. Nader accused Dean of forgetting his maverick role in the Democratic primaries and turning into an apologist for the party.

A third man loomed large in the auditorium at the National Press Club, where the debate took place as part of a radio show. Every inflection and nuance revolved around President Bush.

But it was the sort of debate Republicans welcome: Given that Dean and Nader have nearly identical views on Bush, much of their conversation was about how worthy Sen. John F. Kerry (D-Mass.) is to take on Bush.

Nader has turned into one of the Democrats' biggest irritants this year, at times drawing more ire from party faithful than Bush. Polls nationwide suggest Nader could hold the balance in key states. Democrats fear Nader could tip the election to Bush, as they believe he did four years ago.

Nader has rejected all entreaties to drop out of the race and said yesterday that he would sooner ask Kerry or Bush to drop out than consider withdrawing himself.

Dean, once touted as the Democrats' likely standard bearer in the election, has been considered the party's most potent weapon against the consumer advocate. Dean's antiwar supporters are among the most likely to heed Nader's calls to withdraw U.S. troops from Iraq and slash the defense budget.

The former Vermont governor has pledged to support Kerry and made good on the promise, savaging Nader as the debate got underway. "Ralph, I think you are being disingenuous about your candidacy this year," he began, and listed examples showing that Nader is drawing money and support from groups with agendas the consumer advocate repudiates. In Oregon and Michigan, groups loyal to Bush have organized efforts to get Nader on the ballot, in the expectation that his presence will hurt Kerry. Access to the ballots and the presidential debates are Nader's most daunting challenges -- and his principal goals.

Indiana and Oklahoma are already lost to Nader. Last week, his campaign received a setback in Arizona, and Nader accused Democrats of dirty tricks to keep him off the ballot.

Dean said Nader had benefited from support from the conservative Oregon Family Council. "This campaign of yours is hardly pure," Dean said. He added, "The way to change the country is not to get in bed with right-wing, anti-gay groups."

Nader retorted he had never heard of the Oregon group. Although he is willing to repudiate groups with which he disagrees, Nader said he did not intend to return campaign contributions. "We take all legal donations," he said after the debate. "The only donations we don't take are illegal donations." But Nader's running mate, Peter Camejo, told the San Francisco Chronicle that Nader should return contributions from Republicans who gave with intent of helping Bush.

Nader accused Dean of turning into "Howard Dean II" -- a pale incarnation of the Howard Dean of the Democratic primaries.

"You were an insurgent who has adopted the role of a detergent," Nader said, accusing Dean of washing the Democrats' "dirty linen."

Independent presidential candidates trying to get on the ballot face a complex barrage of arcane state laws. Nader has unleashed a combination of volunteers and paid workers to collect signatures to get him on ballots. Nader defended his workers yesterday for collecting GOP endorsements. To laughter from the audience, he said, "Republicans are human beings, too."

Part of the Nader campaign's measures follow the decision of the Green Party not to endorse him, depriving him of ballot access in 22 states, even though Camejo is a longtime Green activist. Nader did accept the endorsement of the Reform Party, which gives him ballot access in seven states, but also left some of his supporters uneasy.

"I'm very concerned that you are accepting their nomination," Philip Locker told Nader during a recent campaign tour of the Northeast. Locker, who is with a group called Socialist Alternative, said the Reform Party had anti-immigrant and racist policies.

Nader declared then that the Reform Party had "more benign views" and in any case, his acceptance of the endorsement did not mean he supported the group's views. The endorsement was simply a chance to compete in states such as Florida, he told Locker, "unless you want to get 98,000 signatures for us."

During the debate, organized by the National Public Radio show "Justice Talking," the politicians were asked what advice they had for each other. Nader went into a tirade about Dean and the Democrats.

When it was his turn, Dean simply told the consumer advocate: "Lighten up."