WASHINGTON — Senators Clinton and Obama are heading into the final 24 hours before the pivotal Pennsylvania primary by unleashing the heaviest barrage of negative advertising in the Democratic presidential campaign to date, attacking each other regarding tactics as well as their records on health care and ties to Washington lobbyists.

Mr. Obama’s campaign is running a spot criticizing the insurance mandate in Mrs. Clinton’s health care plan, while a Clinton campaign ad questions Mr. Obama’s claim that he does not take money from lobbyists and political action committees; the spot points out that senior staffers in the Illinois senator’s campaign are registered lobbyists and that he has accepted contributions from lobbyists, corporations, and PACs in previous campaigns.

The two campaigns launched additional ads yesterday to rebut the various charges, with Mr. Obama’s commercial labeling Mrs. Clinton’s attacks “eleventh hour smears” and Mrs. Clinton linking the negative ad from Mr. Obama to his debate performance last week.

“He couldn’t answer tough questions in the debate, so Barack Obama is making false charges about Hillary Clinton’s health care plan,” a narrator in the Clinton spot says. “There are more and more questions about Barack Obama,” the narrator says at the conclusion. “Instead of attacking, maybe he should answer them.” The tit-for-tat television war comes as both candidates continue to criticize each other from the stump and as the presumptive Republican nominee, Senator McCain, for the first jumped into the fray over Mr. Obama’s ties to a Vietnam War-era radical, William Ayers.

Mrs. Clinton told voters in Bethlehem, Pa., that Mr. Obama’s campaign had become “so negative,” a day after he had accused her of engaging in “slash-and-burn” politics.

The tone of the campaign leading up to tomorrow’s vote in Pennsylvania may well deepen fears among some Democratic leaders that the protracted primary fight is doing more harm than good to the party’s chances of winning the White House in November.

Most analysts believe that Mrs. Clinton must win the state to keep her nomination hopes alive, with much of the debate focused on exactly how wide her margin has to be to constitute a convincing victory. She had led by more than 15 points at times last month, but two polls released yesterday showed that her edge has dwindled significantly. A Zogby poll has her lead at three points, 46% to 43%, while a Mason-Dixon poll put her ahead by five points, 48% to 43%.

The surveys suggest that Mr. Obama has not been hurt by his much-criticized remarks about small-town Americans at a San Francisco fund-raiser or by his shaky performance in last week’s debate, but they also indicate that as many as 10% of voters have yet to make up their mind.

With all eyes on the Democratic battle, Mr. McCain injected himself into the flare-up over Mr. Obama’s link to Mr. Ayers, a former member of the Weather Underground. The issue came to national attention when Mr. Obama faced questions during Thursday’s debate on ABC about Mr. Ayers, a Chicago neighbor of the candidate who held a fund-raiser and donated to his campaign for the state Senate in Illinois. In 2001, Mr. Ayers was quoted in the New York Times as saying about his involvement with the militant group, “I don’t regret setting bombs; I feel we didn’t do enough.”

Mr. McCain, appearing yesterday on ABC’s “This Week,” brought up Mr. Ayers without prompting when he was asked about Mr. Obama’s patriotism. “I’m sure he’s very patriotic, but his relationship with Mr. Ayers is open to question,” Mr. McCain said, calling Mr. Ayers an “unrepentant terrorist.”

The Arizona senator rebuked Mr. Obama for responding to a question about Mr. Ayers during the debate by comparing him to Senator Coburn of Oklahoma, a staunch conservative with whom he has a friendly relationship. The candidate had said that holding Mr. Obama accountable for statements by Mr. Ayers would be similar to linking Mr. Obama to statements by Dr. Coburn, who has said he favors the death penalty for doctors who perform abortions. “That, in my view, really … it borders on outrageous,” Mr. McCain said.

In the same interview, Mr. McCain defended his decision to accept an endorsement from the Reverend John Hagee, a prominent evangelical who has referred to the Roman Catholic Church as “the great whore” and a “false cult.” Mr. McCain also acknowledged, however, that his campaign had erred in vetting Rev. Hagee and that it was a mistake to solicit his support actively.

A spokesman for the Obama campaign, Bill Burton, fired back at Mr. McCain. “Unable to sell his out-of-touch ideas on the economy and Iraq, John McCain has stooped to the same smear politics and low road that he denounced in 2000,” Mr. Burton said. “The American people can’t afford a third term of President Bush’s failed policies and divisive tactics.”

Mr. Burton also noted that Mr. McCain’s comments signaled a departure from the words five weeks ago of one of his senior advisers, Charles Black. “What Senator McCain has said repeatedly is that these candidates cannot be held accountable for the views of all the people who endorse them or people who befriend them,” Mr. Black said on MSNBC on March 14.