In a tense and grim encounter, the two Democrats were led by their questioners through the words of Obama's former pastor, Clinton's misrepresentation of her visit to Bosnia as first lady, and the links of both presidential contenders to former members of a radical group from decades past.

The candidates also dealt with matters of greater substance, including foreign policy, taxes, Social Security and gun control.

They voiced general agreement on most of those topics, as they have done throughout the race, with both expressing confidence that they could proceed with a relatively rapid withdrawal of U.S. troops from Iraq no matter what the circumstances on the ground there.

Obama and Clinton also disputed the recent assertion of presumptive Republican nominee John McCain that both Democrats would raise taxes on all Americans, saying their plans were limited to revoking the Bush tax cuts for individuals making more than $250,000.

Obama, who seemed on the defensive for much of the conversation about personal vulnerabilities, said that such topics - including the fact that he rarely wears an American flag lapel pin - distract people from dealing with the more pressing matters of economics, health care and foreign policy.

"I have never said that I don't wear flag pins or refuse to wear flag pins," he said in response to a voter's videotaped question. "This is the kind of manufactured issue that our politics has become obsessed with."

Clinton, asked whether she thought Obama could defeat McCain in the general election, replied at first: "We're going to have a Democratic president. It's either going to be Barack or me." Pressed again, she said: "Yes, yes, yes."

She also said jokingly that she had hoped the Republicans would be so ashamed of their performance over the last eight years that they wouldn't run any candidate this time.

But that wasn't going to happen, she said, describing McCain as a formidable opponent and herself as a more electable opponent for him.

"I have a lot of baggage," she conceded, "and everybody has rummaged through it for years."

While much of the debate at the National Constitution Center covered familiar ground, one unlikely and relatively new topic did emerge, involving the long-defunct Weather Underground.

The Illinois senator was asked about his friendship with William Ayres, a former member of the radical group, which was responsible for fatal bombings and other crimes during its heyday four decades ago. In 2001, Ayres expressed no regret for his actions, saying, "I feel we didn't do enough."

"So this kind of game," Obama said last night, "in which anybody who I know, regardless of how flimsy the relationship is, that somehow their ideas could be attributed to me. I think the American people are smarter than that."

The answer did not satisfy Clinton, whose campaign raised the issue several months ago.

"I know Sen. Obama's a good man and I respect him greatly," she said, "but I think that this is an issue that certainly the Republicans will be raising."

And Obama said that Clinton had a problem of her own, because her husband, the former president, had "pardoned or commuted the sentences of two members of the Weather Underground, which I think is a slightly more significant act."

Clinton acknowledged last night that she had been "embarrassed" by her recent incorrect tale of landing under sniper fire in Bosnia during a 1996 trip.

On the matter of Obama's recent remarks describing people in small-town Pennsylvania as bitter, he said he could understand why some people might have been insulted by what he said and how he said it.

But Clinton's efforts to exploit his mistake, Obama said, represented some of what is wrong with American politics.

"The problem with politics is that you take one person's statement, if it's not properly phrased, and beat it to death," he said. "That's the kind of politics we've been accustomed to."

Clinton replied that she wasn't the only person to jump on the remarks, noting that voters and Republicans seized upon them as well.

She said she did not think people "cling to their traditions" such as hunting and guns out of frustration with their government. "I just don't believe that's how people live their lives," she said.

But "what's important is what we both stand for," Clinton said, adding that she believes Pennsylvanians are resilient, positive, and ready for new leadership. "I have a proven record of results."

Although the two showed few differences on policy - and both seemed to struggle explaining their current reluctance to embrace aggressive gun-control measures - they did disagree over what to do about Social Security.

Obama said he would raise the cap on payroll taxes, now set at $97,000, to help make the system solvent. Clinton said that would amount to a tax increase on a lot of people who could not afford it, and said she wanted no tax hikes of any sort for people making less than $250,000.

She said she was committed to saving Social Security through bipartisan measures but did not detail what those measures might be.

The debate, the first since Feb. 26 in Cleveland and the 21st overall for the Democratic candidates, was telecast nationally by ABC and moderated by Charles Gibson and George Stephanopoulos.

Polls in Pennsylvania generally show Clinton leading by 5 to 10 percentage points. Those margins have held steady, more or less, over the last week, despite the flap over Obama's recent comments made at a California fund-raiser.

With Pennsylvania's primary only five days away, both campaigns are gearing up for a final push in the state.

Clinton needs a victory in Pennsylvania, perhaps a decisive victory, to prevent uncommitted superdelegates from rallying to Obama and pushing for an end to the long nomination battle.

With 10 contests remaining, Obama leads Clinton in delegates, popular votes, national polls, and states won - though Clinton has carried many of the larger states.

Depending on Tuesday's outcome, last night's debate may turn out to be the final one between the two.

Clinton has agreed to a debate April 27 in North Carolina, which, along with Indiana, votes May 6. Obama has yet to commit to it.

Clinton in Region

Hillary Rodham Clinton has scheduled at least two appearances in the Philadelphia region today. At 2 p.m. she is to be at Haverford College with her daughter, Chelsea.

The event is closed to the public.

At 8 p.m. the senator is to attend a block party at the Mayfair Diner, 7373 Frankford Ave., Philadelphia.

Barack Obama has no scheduled Pennsylvania events today.

More online coverage of the campaign and debate, including a slide show, at http://go.philly.com/paprimary