PHILADELPHIA -- Democrats Barack Obama and John Edwards sharply challenged Sen. Hillary Rodham Clinton's candor, consistency and judgment Tuesday in a televised debate that underscored her front-runner status two months before the first presidential primary votes.

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-Qaida in Iraq.

"I don't know how you pursue al-Qaida 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.

Some candidates expressed frustration that most of the questions were directed to Clinton, Obama and Edwards. Seventeen minutes into the debate, Ohio Rep. Dennis Kucinich had yet to get a question and blurted out, "Is this a debate here?" Minutes later, New Mexico Gov. Bill Richardson 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 Dodd 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. Joe Biden said former New York Mayor Rudy Giuliani "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.

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 Mitt Romney 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. Mike Gravel on grounds that he did not meet fundraising and polling thresholds.

Democrats Quoted

Joe Biden On education: "We should go to school longer. ... Secondly, we should have a minimum 16 years of education. Thirdly, we should be focusing on the socio-economic disadvantaged, mostly minorities in inner cities. That's something we've ignored. We pay no attention to it. We pretend they're in the same circumstances as every other kid in America. They start off with half. Half of the education gap exists before they set foot in the first classroom. That should be the focus."

Hillary Clinton on Republican attacks: "In a perverse way, I think that the Republicans and their constant obsession with me demonstrate clearly that they obviously think that I am communicating effectively about what I will do as president. I am trying to do that because it matters greatly. We've got to turn the page on George Bush and Dick Cheney. In fact, we have to throw the whole book away. This has been a disastrous period in American history, and we hope it will be an aberration."

Chris Dodd on the passage of a resolution designating Iran's Revolutionary Guard as a terrorist group: "I believe that this issue is going to come back to haunt us. We all learned ... back in 2002, that by voting for an authorization regarding Iraq, that despite the language of that resolution which called for diplomacy at the time, this administration used that resolution, obviously, to pursue a very aggressive action in Iraq. ... What you didn't learn back in '02, you should've learned by now."

John Edwards on energy independence: "We need to ask Americans to be patriotic about something other than war. And I want to be the president who says to America, we're in this together. We're going to have to be willing to sacrifice. If we love this country enough, we're going to have to conserve, in our homes, in our workplaces, and alter our behavior to make America what it's capable of being."

Dennis Kucinich on nuclear weapons: "It is time that the United States government enforced and participated in fully the Nonproliferation Treaty, which calls for the abolition of all nuclear weapons. We must lead the way, and we must have a president who understands the danger of these nuclear weapons and have America lead the way among all nations towards nuclear abolition. When we do that, we will have the credibility to go to an Iran and any other nation that may have desires for nuclear power to say, Look, we want to take it in another direction."

Barack Obama on his background: "There is no doubt that my background is not typical of a presidential candidate. I think everybody understands that. But that's part of what is so powerful about America, is that it gives all of us the opportunity - a woman, a Latino, myself - the opportunity to run. ... As we travel all across the country, we have received enormous support, in states where, frankly, there aren't a lot of African-Americans, and there aren't a lot of Obamas."

Bill Richardson on education: "I have a specific proposal. Here it is: In exchange for two years of tuition paid by the government or loans, you give one year of national service to the country. This will attract more doctors and will enable students to afford a college education when it's taking them seven years to pay for this. Get rid of the student loan and bank agencies that are ripping off the system.