Senator Obama is escalating his criticism of Senator Clinton’s record on the Iraq war, using a Democratic presidential debate last night to belittle her attempt to force the Pentagon to release plans for withdrawing American troops.

“I think it’s terrific that she’s asking for plans from the Pentagon, and I think the Pentagon response was ridiculous,” the Illinois senator said in a response unprompted by a specific question about Mrs. Clinton, before targeting her initial support for the Iraq invasion: “But what I also know is that the time for us to ask how we were going to get out of Iraq was before we went in.”

Mr. Obama’s unsolicited jab, perhaps his most direct of the campaign, signals a potential turning point in a race that has thus far seen the leading contenders reluctant to take on their opponents directly. His statement came moments after Mrs. Clinton boasted of her recent fight with a high-ranking Bush administration official who had suggested in a letter that she was “reinforcing enemy propaganda” by stirring talk of an American exit from Iraq.

The former first lady was not given a chance to respond to Mr. Obama’s shot, but she appeared to hit back at his lack of foreign policy experience minutes later when she pointedly refused to follow him in promising to meet personally with the leaders of enemy states and sponsors of terrorism — including Cuba, Venezuela, North Korea, and Iran — in his first year in office.

“I will not promise to meet with the leaders of these countries during my first year,” Mrs. Clinton said, adding that she would pursue a “vigorous diplomatic effort” but warning against high-level meetings “before you know what the intentions are.”

“I don’t want to be used for propaganda purposes. I don’t want to make a situation even worse,” she said, naming Fidel Castro of Cuba and President Chavez of Venezuela as leaders she would not meet “until we know better what the way forward would be.”

Sensing an opportunity to underscore Mrs. Clinton’s message of “strength and experience,” her campaign highlighted the exchange in an e-mail to reporters before the event had ended.

The debate, held at the Citadel in Charleston, S.C., was sponsored by CNN and YouTube and featured questions from Americans who filmed themselves on 30-second homemade videos they uploaded to the video-sharing site. CNN chose the video questions, picking a combination of substantive policy queries along with several that were light-hearted and a few that were simply awkward, such as one that featured an animated snowman asking a question about global warming.

Mrs. Clinton and a former North Carolina senator, John Edwards, tiptoed around comments that his wife, Elizabeth, had made last week criticizing Mrs. Clinton’s record on women’s issues and saying her husband would be a better advocate for women. Mrs. Clinton recited what she said was a long history of supporting causes important to women, while Mr. Edwards, standing beside her on the stage, praised her record and refused to say directly whether his wife was correct.

Along the way, the questions deepened rifts among the eight declared candidates, particularly on foreign policy. Mrs. Clinton has gone to great lengths during the campaign to court the party’s anti-war base by calling increasingly for a withdrawal of troops from Iraq. But she appeared to draw the line when pressed by Governor Richardson of New Mexico, who acknowledged he was “trying to provoke a debate here” when he criticized her plans to keep residual forces in the country; he has pledged to pull out every single American soldier as president.

“With all due respect to some of my friends here, yes, we want to begin moving the troops out, but we want to do so safely, and orderly and carefully,” Mrs. Clinton said, her voice almost pleading. “We don’t want more loss of American life and Iraqi life as we attempt to withdraw, and it is time for us to admit that it’s going to be complicated, so let’s start it now.”

Her comments followed even stronger language from Senator Biden of Delaware, who chastised his fellow candidates in the Senate, including Mrs. Clinton, for voting against an emergency funding bill for Iraq because it did not include a timetable for troop withdrawal. He said they were voting against sending critical supplies, including armored vehicles, to the front lines. “How in good conscience can you vote not to send those vehicles over there as long as there’s one single, solitary troop there?” Mr. Biden said.

The Delaware senator and Mrs. Clinton also presented different positions on ending the Darfur genocide. Mr. Biden said he would “absolutely” send American troops to intervene, while Mrs. Clinton, after prodding from moderator Anderson Cooper, said they “don’t belong in Darfur at this time.”

As the front-runner in most polls, Mrs. Clinton fielded several tricky and pointed questions throughout the night. She deflected a few of them, with mixed success. She refused to label herself as a “liberal,” choosing the term “modern progressive,” and when pressed by a questioner skeptical of four straight Bush or Clinton administrations, she quipped, “I think it is a problem that Bush was elected in 2000.”