PHILADELPHIA -- Buffeted by attacks from her rivals for the Democratic nomination for president, Sen. Hillary Rodham Clinton defended her Senate record and her front-runner status last night as the seven leading contenders debated topics ranging from Iran to UFOs.

Illinois Sen. Barack Obama and former North Carolina Sen. John Edwards flanked Mrs. Clinton on the debate stage and took turns from the outset assailing the New York senator's record and potential to be an effective nominee.

"She defends a broken system," Mr. Edwards said, trying to characterize her as a creature of the status quo in Washington.

Mr. Obama contended that the Democratic front-runner had repeatedly changed her position on crucial issues ranging from trade to the war in Iraq. But the sharpest exchanges in the early portion of the debate focused on Mrs. Clinton's vote in favor designating Iran's Revolutionary Guard as a terrorist organization.

"She voted to give George Bush the first step in moving militarily against Iran," Mr. Edwards said of the Senate vote last month.

Two of her rivals, Sens. Christopher Dodd, D-Conn., and Joseph Biden, D-Del., voted against the Iran legislation while Mr. Obama did not cast a vote on it. The issue sparked a near rerun of the controversy over the legislation that authorized the use of force against Iraq. Mrs. Clinton and other defenders of the measure contend that it is a focused, limited bill. Critics, however, argue that it could be used by the Bush administration as a green light for escalation of U.S. action against Tehran.

"This has incredible consequences for Afghanistan and Pakistan," said Mr. Biden. "[It] plays into this whole urban legend that American is on a crusade against Islam ... all it has done is hurt us."

"The way you put pressure on this administration is you stand up to them, you say no," Mr. Edwards added.

Mrs. Clinton, appearing unfazed by the widely anticipated attacks, rejected those assertions and described the controversial measure as part of a complex diplomatic approach to containing Iraq's nuclear ambitions.

"We have to try diplomacy, and I see economic sanctions as part of diplomacy."

"I'm against a rush to war," Mrs. Clinton said at another point. "I am not in favor of this rush to war, but I am also not in favor of doing nothing ... I prefer vigorous diplomacy and I happen to think the economic sanctions are part of vigorous diplomacy."

Mr. Edwards also faulted Mrs. Clinton for not advocating a speedier withdrawal from Iraq.

"I stand for ending the war in Iraq and bringing our troops home," she responded, "but I also understand that it's going to take time."

The focus on Mrs. Clinton came after months in which national polls showed her as the consistent commanding leader among national Democrats.

An aggregate of surveys compiled by the Web site Pollster.com showed her with an average 44.6 percent among Democrats, far ahead of Mr. Obama's 19 percent and Mr. Edwards' 11 percent.

In Iowa, polls have showed a tighter race and the trailing Democrats hope that an upset there could reshuffle the standings.

Mr. Edwards had led in Iowa for much of the year but the consensus of recent surveys suggests that both Mrs. Clinton and Mr. Obama are now vying for the lead.

The most recent Pollster.com average for Iowa showed Mrs. Clinton at 30 percent, Mr. Obama, 24 percent and Mr. Edwards at 19 percent.

Rivals of Mrs. Clinton -- and some analysts -- have pointed to former Vermont Gov. Howard Dean as a cautionary example for anyone who projects Mrs. Clinton as a prohibitive favorite.

At this point in the election cycle four years ago, Mr. Dean had a big lead in money and polls, only to fade before the Iowa caucuses.

Mr. Dean, however, who appeared last night in his role as Democratic National Committee chairman, was an insurgent candidate four years ago, with a new, and to public perception, malleable image. Mrs. Clinton has the support of much of the Democratic establishment. And her image with the public, positive or negative, is grounded in years of prominence. The respondents to the various polls have had years to make up their minds about her.

Mr. Obama kidded about his underdog status in the opening moments of the debate, along with the predictions that it would force him to play the aggressor against Mrs. Clinton.

"Some of that stuff gets over-hyped," he said. "This is the most over-hyped fight since Rocky fought Apollo Creed -- although I'm Rocky in this situation."

Despite Mrs. Clinton's popularity with Democrats, Mr. Dodd depicted her as a polarizing figure who would give the Republicans the best chance of holding onto the White House.

Echoing that contention, Mr. Obama said, "Part of the reason Republicans are obsessed with you, Hillary, is I think that's a fight they're very comfortable having. What we don't need is another eight years of bickering."

"Another perspective on why the Republicans keep talking about Sen. Clinton is they may actually want to run against you," echoed Mr. Edwards.

As the volleys against the front-runner continued, New Mexico Gov. Bill Richardson came to her defense, bemoaning his fellow candidates' "holier than thou attitudes," and "personal attacks" toward Mrs. Clinton.

Mr. Obama dismissed suggestions that his name, sometimes deliberately or accidentally confused with the loaded appellation, Osama, was symbolic of his own electability problems.

He said that he was confident that he would not be vulnerable to unfair attacks, "because I have confidence in the American people. The way to respond to swift boating is to respond forcefully, rapidly, and truthfully, and I have absolute confidence in the American people's capacity to absorb the truth."

Republicans came in for plenty of criticism during the two-hour session as well.

Rep. Dennis Kucinich, D-Ohio, insisted that the war in Iraq had been about nothing but control of oil and repeated his call for impeachment against the president and Vice President Dick Cheney for leading the country into that war.

Turning to the early front-runner among the Republicans hoping to succeed Mr. Bush, Mr. Biden derided the qualifications of former New York City Mayor Rudy Giuliani, calling him the most unqualified presidential candidate since the incumbent.

In response to one question, Mr. Kucinich acknowledged a report that he had seen a UFO.

Smiling, he noted that former President Jimmy Carter had once said that he had seen a UFO, and added that, "More people in this country have seen a UFO than approve of Mr. Bush's presidency."

The debate, the seventh among the leading Democrats, took place on the campus of Drexel University in West Philadelphia.

The two-hour event was broadcast live on MSNBC. The candidates were questioned by NBC hosts Brian Williams and Tim Russert.