PHILADELPHIA -- The Democratic presidential contenders debated issues ranging from honesty to patriotism to their plans for Social Security and Iraq last night in a tense and humorless encounter days before the crucial Pennsylvania primary.

Sen. Barack Obama was repeatedly placed on the defensive through the first portion of the debate, as the moderators of ABC and his opponent pressed him on questions that have dogged his campaign in recent weeks, including the controversial statements of his former pastor and his characterizations of small-town voters.

Mr. Obama argued that the focus of such questions was a distraction from more serious problems.

"It would be pretty hard of me to be condescending to people of faith when I am a person of faith," he said at one point. "The problems we have in our politics -- you take one person's statements, if it's not properly phrased and you just beat it to death."

Mrs. Clinton was happy to add fuel to that rhetorical fire as she called Mr. Obama "a good man," but questioned his decision to remain in a church presided over by his former pastor, the Rev. Jeremiah Wright.

"I have to say that for Pastor Wright to have given his first sermon after 9/11 and to blame the United States for the attack, which happened in my city, would have been intolerable to me ... you get to choose your pastor, you don't get to choose your family."

Later, she added, "As leaders we have a choice of who we associate with, and who we apparently give some kind of seal of approval to. And I think that it wasn't only the specific remarks, but the relationship with Rev. Farrakhan, with giving the church bulletin over to the leader of Hamas to put a message in. These are problems, and they raise questions in people's minds."

Mr. Obama recalled that when her husband first ran for president, Mrs. Clinton had been forced to answer critics over her statement that she didn't choose to sit home and bake cookies. He said that he realized at the time that that was not an expression of her true self.

He warned against becoming "so obsessed with gaffes that we lose sight of the real stakes in the election."

But the greatest hits in gotcha moments continued as, in response to another questions, Mrs. Clinton once again apologized for her false accounts of landing in Bosnia under sniper fire. She noted that she had described the incident accurately in the past, but somehow lapsed into the more colorful version that she has since apologized for.

"I may be a lot of things, but I'm not dumb ... I'm embarrassed by it; I've apologized for it. I've said it was a mistake."

Mr. Obama seemed exasperated when one of the ABC moderators asked him about his association with William Ayers, a former member of the Weather Underground who lives in his Chicago neighborhood and who has been quoted as saying that he had no regrets for his actions in the radical group in the 1960s.

After Mr. Obama minimized their association, expressing incredulity that he would be questioned about a casual acquaintance "who engaged in detestable acts 40 years ago when I was 8 years old."

Mrs. Clinton wasn't ready to let it rest.

"I also believe that Sen. Obama served on a board with Mr. Ayers for a period of time, the Woods Foundation, which was a paid directorship position. ... I know Sen. Obama is a good man, and I respect him greatly. But I think this is an issue that certainly the Republicans will be raising."

Mr. Obama shot back, "President Clinton pardoned or commuted the sentences of two members of the Weather Underground, which I think is a slightly more significant act than me serving on a board with somebody for actions that he did 40 years ago when I was 8 years old."

With a question from a viewer, Mr. Obama was asked about the charge, widely spread on the Internet, that he is reluctant to honor the flag.

"I revere the American flag and I would not be running for president if I did not revere this country," he said, adding, "There is no other country in which my story is even possible. ... I could not help but love this country for all that it's given me."

The debate turned from the personal to economic and foreign policy issues in its second half as the two senators traded views on Iraq, Social Security and gun control. The debate was the 21st of the campaign and their first in more than six weeks. It followed weeks of campaign melodrama in which one and then the other candidate found themselves on the defensive.

Mr. Obama rebounded, at least temporarily, from the controversy over the incendiary sermons of the Rev. Wright, in a widely praised speech on race and politics delivered a month ago in the same building where he debated last night. That story soon had to share the news cycle with Mrs. Clinton's backtracking over her inaccurate accounts of a landing in Bosnia under sniper fire during her husband's administration. That embarrassment made the Clinton campaign doubly glad to change the subject with the emergence of the controversy over Mr. Obama's reflections on "bitter" small-town residents who "cling to" religion and guns.

The Clinton campaign jumped on the remark like a fumble, but by mid-week there was little evidence that it had engaged the public as much as it had the commentariat. Several published polls showed Mrs. Clinton holding onto a lead in the state but one that had eroded substantially since the race turned to Pennsylvania. Even more damaging to Mrs. Clinton's overall nomination hopes, the flap did not create any tidal shift in the movement of superdelegates.

Mr. Obama picked up two superdelegates yesterday, these from North Carolina, the site of one of the next nomination contests after Pennsylvania. Mrs. Clinton is counting on Pennsylvania to allow her to claim a big-state trifecta after her big Ohio win and a popular-vote victory in Texas that allowed her to reclaim a measure of momentum after Mr. Obama's 11-state winning streak through February.

The New York senator has virtually no hope of overtaking Mr. Obama in delegates elected in primaries and caucuses, but she hopes that a series of big-state losses will convince the party's superdelegates that he is unelectable in the fall.

The invited guests and members of the press entered the National Constitution Center through an exuberant phalanx of partisans of each of the campaigns. Independence Mall and the adjacent street corners were filled with sign-waving demonstrators who competed with shouts of "Hill-a-ry, Hill-a-ry," and the Obama camp's signature chant, "Fired up -- ready to go."