Senator Clinton delivered a more focused and energetic performance than Senator Obama at a high-stakes presidential debate last night, but she failed to deliver any knockout punch or to benefit from any gaffe likely to upend Mr. Obama’s juggernaut toward the Democratic nomination.

The first half of the 90-minute CNN/Univision debate produced a polite discussion of policy on Cuba, immigration, and other issues but illuminated few differences between the candidates. At that point, the moderators began trying to ignite some fireworks, but the sparks flew only after Mr. Obama was asked to respond to claims by Mrs. Clinton’s campaign that he plagiarized portions of his stump speeches from Governor Patrick of Massachusetts.

“It’s not a lot of speeches. There are two lines in speeches that I’ve been giving over the last couple of weeks,” Mr. Obama said. “The notion that I had plagiarized from somebody who was one of my national co-chairs who gave me the line and suggested that I use it, I think, is silly, and you know, this is where we start getting into silly season, in politics, and I think people start getting discouraged about it.”

“I think if your candidacy is going to be about words, then they should be your own words,” Mrs. Clinton replied. “That’s I think a very simple proposition. Lifting whole passages from someone else’s speeches is not change you can believe in, it’s change you can Xerox,” she said, delivering the most memorable line of the night, but also eliciting a chorus of boos from the audience at the University of Texas at Austin.

“That’s not what happened,” Mr. Obama insisted.

“You know what, Barack, it is,” she said, as the booing continued. “If you look at the YouTube of these videos, it does raise questions.”
The debate’s only significant development on the policy front was a pledge by Mrs. Clinton to move quickly to introduce immigration legislation if she becomes president. “There must be a path to legalization. I would introduce that in the first 100 days of my presidency,” While campaigning in Iowa in December, the former first lady refused to make that commitment, which effectively one-ups Mr. Obama’s promise to push such legislation within a year of assuming the presidency.

The urgency both candidates have pledged on the issue is a rebuke to a prominent Democratic leader in the House, Rep. Rahm Emanuel of Illinois, who has reportedly said the issue should not be considered in the first term of a new Democratic president.

On a couple of occasions last night, the former first lady showed flashes of emotion, something some of her advisers began counseling her to do again after she teared up at a New Hampshire café. As the debate concluded, she grew emotional as she said, “Whatever happens, we’re going to be fine. … I just hope that we’ll be able to say the same thing about the American people.” She was also passionate as she complained that Mr. Obama’s health care plan lacked a so-called mandate and would leave millions of Americans without insurance. “This is too important. This is the number one issue that people talk to me about. You know, when a mother grabs my arm and says, ‘I can’t get the operation my son needs because I don’t have health insurance,’ it is personal for me,” she said.

Mr. Obama said mandates don’t work well, as is being seen with such an effort in Massachusetts. “They have exempted 20% of the uninsured because they have concluded that that 20% can’t afford it,” he said. “There are people who are paying fines and still can’t afford it, so now they’re worse off than they were.”

On Iraq, Mrs. Clinton acknowledged that the troop surge in Iraq has reduced violence, but said that “the purpose of it has not been fulfilled” by producing political progress in the Iraq government.

“This is a tactical victory imposed on a huge strategic blunder,” Mr. Obama said, before turning the question into a chance to argue that he would be a more formidable challenger to the presumptive Republican nominee, Senator McCain of Arizona. “When we’re having a debate with John McCain, I think it is going to be much easier for the candidate who was opposed to the concept of invading Iraq in the first place to have a debate about the wisdom of that decision than having to argue about the tactics subsequent to that decision.”

Asked to identify the moment in their lives that had tested them the most, Mr. Obama essentially passed, missing an opportunity to allay fears that he is not sufficiently experienced for the presidency.

“I think everybody here knows I’ve lived through some crises and some challenging moments in my life,” Mrs. Clinton said, alluding vaguely to the sex-and-lies scandal that almost drove her husband from office.