An unusually sharp exchange among the leading Democratic presidential campaigns could signal that the often courtly tone of the race thus far is about to give way to a more confrontational, negative approach.

In a television interview billed as a preview of tonight’s debate in South Carolina, the campaign manager for John Edwards, David Bonior, called Senator Clinton’s record on health care issues “an absolute disaster.”

Minutes later, an Alabama congressman speaking for Senator Obama of Illinois, Rep. Artur Davis, said rather directly that a vote for Mrs. Clinton would resurrect the disputes that tarred the presidency of her husband, President Clinton.

“It’s past time to take the gloves off on the Democratic side,” a professor of politics at the University of Virginia, Larry Sabato, said. “It’s amazing that we’ve made it so far without seeing more of it.”

The first punch on CNN’s “Late Edition” yesterday was thrown by Mr. Bonior. He interrupted a former Iowa governor representing Mrs. Clinton, Thomas Vilsack, to challenge his claim that the former first lady had “delivered” on health care by spearheading efforts to put children on government-funded insurance.

“With all due respect … the Clintons did not deliver on health care,” Mr. Bonior said. “They had a very important choice to make back in ’93: whether to do the North American Free Trade agreement or health care. They implemented the North American Free Trade Agreement that put literally millions of workers out of work in this country and destroyed, basically, our good trading relationships we had around the world. And then in the interim, they lost any capital they had to get health care passed. … The fact of the matter is it’s been an absolute disaster on health care.”

The intensity of the double-barreled attack on both trade and health care seemed to take Mr. Vilsack by surprise. The Iowa governor responded by suggesting that Mrs. Clinton’s accomplishments outshined any of her rivals.

“I’ve seen the impact and effect of the health-care work that Hillary Clinton did in terms of the children’s health insurance program. That is real. That is real relief to families that matters,” Mr. Vilsack said. The back-and-forth gave Mr. Davis an opening to paint his candidate as an antidote to what some call Clinton fatigue.

“The last exchange you’ve heard is exactly why Barack Obama’s candidacy is so important. We’ve just spent two minutes arguing about who did what in 1993,” Mr. Davis said. “Democrats will not win the election in 2008 if we are frozen in an argument about who did what in 1993, who did what between 1993 and 2001. That’s a stale argument for a lot of people. As much as I admire the Clintons, people are hungering for a new discussion in this country, and they want to look forward.”

While pundits, rank-and-file Democrats, and occasionally fund-raisers have leveled similar critiques at Mrs. Clinton, political analysts said it was unusual to see such direct public attacks by top-level leaders of the competing campaigns.

“This is different. These people matter,” Mr. Sabato said. “It’s finally suggesting that her little cakewalk may be over.”

“As we get closer to January, we’re going to find more and more of these comparisons, it’s just natural,” Norman Ornstein of the American Enterprise Institute said. “We’re likely to see far more coming from surrogates than from candidates. They always like to stay above the fray on this stuff. That’s particularly true if they can get attention from the press for what the surrogates say.”

Just a few days earlier, Elizabeth Edwards grabbed headlines with her suggestion that Mrs. Clinton was sidestepping women’s issues to look manlier.

Mr. Sabato said candidates like Messrs. Obama and Edwards, as well Governor Bill Richardson of New Mexico, will effectively cede Mrs. Clinton the nomination if they don’t quickly take her on. “If Obama, Edwards, and Richardson don’t start going after Hillary Clinton soon, it’s over. They have to make exactly these arguments, that to nominate her is to rehash the 1990s and all the Clinton controversies,” the professor said.

An ABC News-Washington Post poll released today showed Mrs. Clinton with 45% support among Democrats and Democratic-leaning independents, followed by Mr. Obama with 30% and Mr. Edwards with 12%. Mr. Richardson, who has shown some momentum in Iowa and New Hampshire polls recently, scored just 3% in the national survey.

Mr. Obama’s support is strongest with wealthier and better educated voters, as well as independents. However, Mrs. Clinton is doing roughly as well with these groups, the poll shows.

On another Sunday talk show yesterday, Senator Feingold, a Democrat of Illinois, renewed his call for Congress to censure President Bush and Vice President Cheney. “This administration has assaulted the Constitution,” Mr. Feingold told NBC’s “Meet the Press.”

Mr. Feingold billed his latest censure plan as “a moderate course,” short of impeachment. “There’s a lot of sentiment in the country, even the polls show it, for actually impeaching the president and the vice president,” the senator said.

Senator McConnell, a Republican of Kentucky, said the tactic was politically foolish and akin to the overnight Senate session just held about Iraq. “The kind of stunt that Senator Feingold just recommended on the heels of the all-night theater of Tuesday night gives you a sense … of why this Congress now has a 14% approval rating,” Mr. McConnell told CNN. “All they do is have Iraq votes and investigations.”