WASHINGTON — A Democratic race that has taken on a nasty tone in recent weeks may be returning to civility following a debate last night in which senators Clinton and Obama sparred substantively over health care policy, immigration, and the Iraq war.

Looking to make up ground before more than half the nation heads to the polls February 5, Mr. Obama criticized Mrs. Clinton’s judgment in voting for the Iraq war in 2002, saying her pledge to end the conflict as president was not sufficient.

“I don’t want to just end the war,” Mr. Obama said. “I want to end the mindset that got us into the war in the first place.”

Mrs. Clinton was forced to explain again her decision to vote in favor of authorizing the war, but she faced a harsh retort not from Mr. Obama but from the debate moderator, Wolf Blitzer of CNN, who suggested she was admitting to being “naïve” by giving President Bush the power to invade Iraq.

“That’s not what you heard me say. Good try, Wolf,” Mrs. Clinton replied, to laughter in the audience at the Kodak Theatre in Hollywood.

Mr. Obama did not say Mrs. Clinton was naïve, but he rejected her explanation nonetheless, saying everyone understood that the vote could send the nation to war.

The debate was the first one-on-one face-off between Mrs. Clinton and Mr. Obama following the departure from the race of John Edwards on Wednesday. Both candidates praised the former North Carolina senator and worked in pledges to take on his signature issue — poverty — in what appeared to be open appeals for his endorsement, and for the votes of those who supported him.

The tone of the encounter was markedly different from the last Democratic debate, which descended into a series of scathing and personal attacks. Mr. Obama used his opening statement to laud Mrs. Clinton. “I just want to note that I was friends with Hillary Clinton before this campaign started. I will be friends with Hillary Clinton after this campaign,” he said, in remarks later echoed by Mrs. Clinton.

The candidates delved deeply into their plans for universal health care, with Mr. Obama criticizing Mrs. Clinton for including a mandate that would force all Americans to buy insurance. His plan requires coverage for children, but not for adults, and he questioned how Mrs. Clinton would enforce her requirement. “If they cannot afford, the question is, what are you going to do about it? Are you going to fine them? Are you going to garnish their wages?” he asked. “Those are questions that Senator Clinton has not answered with respect to her plan, but I think we can anticipate that there would also be people potentially who are not covered and are actually hurt if they have a mandate imposed on them.”

Mrs. Clinton in turn accused Mr. Obama of taking the easy way out by not including a mandate, saying his plan would leave as many as 15 million people uninsured. She also pointed out that his proposal does not require children to be covered. “So it’s not that he is against mandatory provisions, it’s that he doesn’t think it would be politically acceptable to require that for everyone. I just disagree with that,” Mrs. Clinton said, adding that a Democratic plan that did not start as “universal” would be “nibbled to death.”

Mrs. Clinton and Mr. Obama also tangled over immigration, with each accusing the other of switching their position over whether illegal immigrants should be eligible for driver’s licenses. After Mr. Obama suggested that Mrs. Clinton and other Democrats had not taken a strong stand in advocating comprehensive immigration reform, Mrs. Clinton pointed that she had introduced the first such bill in 2004, “before Barack came to the Senate.”

The two focused more of their barbs on Republicans, particularly on the party’s front-runner, Senator McCain. Mr. Obama drew laughs when he said that Mr. McCain’s famed “Straight Talk Express” had “lost some wheels” because the Arizona senator is now calling for President Bush’s tax cuts to be made permanent after he voted against them in 2001 and 2003.

While Mr. Obama has said in recent days that the contest between him and Mrs. Clinton about “the past versus the future,” he was careful not to criticize directly her husband’s tenure, saying “there were good things” during President Clinton’s two terms in office.

The former first lady defended Mr. Clinton’s record forcefully, and she drew the loudest cheer of the night when she batted down a question suggesting that she would not represent much of a change after five consecutive terms of a Bush or a Clinton as president. “It did take a Clinton to clean up after the first Bush and I think it might take another one to clean up after the second Bush,” she said to sustained applause, in a variation of a line she has used on the campaign trail.

Despite several key endorsements going to Mr. Obama in the last week, Mrs. Clinton has maintained a lead in national polls and in the largest of the 22 states that vote on February 5, including New York, New Jersey, and California.