Obama, the Illinois senator, began immediately, saying Clinton has changed her positions on the North American Free Trade Agreement, torture policies and the Iraq war. Leadership, he said, does not mean "changing positions whenever it's politically convenient."
Edwards, the former North Carolina senator, was even sharper at times, saying Clinton "defends a broken system that's corrupt in Washington, D.C." He stood by his earlier claim that she has engaged in "doubletalk."
Clinton, standing between the two men, largely shrugged off the remarks and defended her positions. She has been the focus of Republican candidates' "conversations and consternation," she said, because she is leading in the polls.
She said she has specific plans on Social Security, diplomacy and health care. "I have been standing against the Republicans, George Bush and Dick Cheney," she said, "and I will continue to do so, and I think Democrats know that."
It was the Democrats' first debate in a month, and during that time Clinton has solidified her front-runner position, gaining in polls, taking the lead in fundraising and dominating the agenda. The Iowa caucuses are scheduled for Jan. 3, and the New Hampshire primary could be even earlier.
Clinton defended her Senate vote in favor of designating Iran's Revolutionary Guard as a terrorist group. Obama, Edwards and others have said President Bush could interpret the measure as congressional approval for a military attack.
Edwards caustically challenged Clinton's claim that she stands up to the Bush administration. "So the way to do that is to vote yes on a resolution that looks like it was written literally by the neocons?" he said.
"In my view, rushing to war - we should not be doing that - but we shouldn't be doing nothing," Clinton said. "And that means we should not let them acquire nuclear weapons, and the best way to prevent that is a full court press on the diplomatic front."
Clinton also was the main focus during a discussion of the Iraq war. Again, Edwards leveled the toughest charges against the New York senator.
"If you believe that combat missions should be continued in Iraq" without a timetable for withdrawal, Edwards said, "then Senator Clinton is your candidate." Edwards vowed to have all combat troops out of Iraq "in my first year in office."
Clinton replied forcefully, saying "I stand for ending the war in Iraq, bringing our troops home." She added, however, that "it is going to take time," and some troops must remain to fight al Qaeda in Iraq.
"I don't know how you pursue al Qaeda without engaging them in combat," she said.
Edwards, drawing a link between Iraq and Iran, pressed on. "What I worry about is, if Bush invades Iran six months from now, I mean, are we going to hear: 'If only I had known then what I know now?"' He was alluding to comments Clinton has made about her 2002 vote to authorize military action against Saddam Hussein.
"Edwards and Obama achieved at least some of what they wanted to do coming into this debate and that was to exploit some of Clinton's vulnerabilities with the Democratic base, especially on Iran and Iraq," said CBSNews.com Senior Political Editor Vaughn Ververs. "Clinton defended herself well, but the criticism may play well with voters already uneasy with her past support of the war in Iraq." (Read more debate analysis from Vaughn Ververs in Horserace.)
Some candidates expressed frustration that most of the questions were directed to Clinton, Obama and Edwards. Seventeen minutes into the debate, Ohio Rep. had yet to get a question and blurted out, "Is this a debate here?" Minutes later, New Mexico Gov. threw up his hands in protest that he hadn't been called on either and exchanged a frustrated glance with Kucinich.
Obama, alluding to the partisanship that bedeviled Bill Clinton's presidency, told the former first lady: "Part of the reason that Republicans, I think, are obsessed with you, Hillary, is because that's a fight they're very comfortable having. It is the fight that we've been through since the '90s."
Richardson criticized his rivals for challenging Clinton so sharply, rebuking their "holier-than-thou attitude."
But Edwards and Connecticut Sen.cited Clinton's relatively high unfavorability ratings.
"Fifty percent won't vote for her," Dodd said.
On Social Security, Russert asked Clinton why she told an Iowa voter, in an offstage comment overheard by an Associated Press reporter, that she was open to raising the cap on payroll taxes when the proposal is not part of her platform.
Clinton said she did not have a "private position" on Social Security. She would convene a bipartisan commission to recommend ways to strengthen the program, she said, and all the well-known suggestions "would be considered."
Only briefly did the candidates aim their remarks at Republicans. Delaware Sen.said former New York Mayor "is genuinely not qualified to be president."
Giuliani's entire message is "a noun, a verb and 9/11," Biden said, but that he had "done nothing" to implement anti-terrorism recommendations by the 9/11 Commission.
Edwards, meanwhile, felt at least one jab. Kucinich, alluding to Edwards' past financial dealings, said: "When people get money from New York hedge funds and then they attack another person for getting money from Washington interest groups, you know what? They're both right."
Clinton said a New York state proposal to give drivers' licenses to illegal immigrants "makes a lot of sense," but she stopped short of a wholehearted endorsement. Only Dodd said he flatly opposed the idea.
CBS News senior political correspondent Jeff Greenfield said that was probably the most uncomfortable moment for Clinton during the debate.
"We're going to find out if that pretty clear stumble was just a flicker or something more, but I do think this was the first debate I watched where Senator Clinton seemed a bit off her game," Greenfield said on CBS News' The Early Show.
In the debate's lightest moment, Kucinich confirmed seeing an unidentified flying object at the Washington state home of actress Shirley MacLaine. He said, with a smile, he would open a campaign office in Roswell, N.M., home to many alleged UFO sightings.
Obama said he would accompany his daughters in trick-or-treating on Halloween while wearing a mask, which has "two sides to it, it goes in both directions."
The debate, held at Drexel University, was aired by MSNBC. Organizers excluded former Alaska Sen.