Senator Obama of Illinois is emerging as the punching bag of the moment in the Democratic presidential race, after bearing the brunt of criticism from his rivals during a debate yesterday in Iowa.

The front-runner for the Democratic nomination in national polls, Senator Clinton, seemed eager to keep Mr. Obama at bay, while other candidates clamored to seize the political newcomer’s mantle as the most plausible alternative to the former first lady.

Mrs. Clinton kicked off the attacks by questioning the wisdom of Mr. Obama’s pledge to meet one on one with leaders of hostile regimes such as Cuba, North Korea, and Iran. “I do not think that a president should give away the bargaining chip of a personal meeting with any leader, unless you know what you’re going to get out of that,” she said. “You should not telegraph to our adversaries that you’re willing to meet with them without preconditions during the first year in office.”

Another hopeful, Senator Dodd of Connecticut, piled on, faulting Mr. Obama for declaring that America should attack Al Qaeda terrorists inside Pakistan even if President Musharraf objects. “It was irresponsible to engage in that kind of a suggestion here. That’s dangerous,” Mr. Dodd said. “You’re not going to have time in January of ’09 to get ready for this job.”

The debate’s moderator, George Stephanopoulos of ABC News, prompted some of the attacks by opening the 90-minute exchange with a question about whether Mr. Obama is ready for the presidency.

Mr. Obama acknowledged the flurry of criticism with a quip. “Well, you know, to prepare for this debate, I rode in the bumper cars at the state fair,” he joked, drawing much laughter from the audience.

Mr. Obama parried Mrs. Clinton’s critique by suggesting that she was adopting President Bush’s strategy of freezing out enemy leaders. “Strong countries and strong presidents meet and talk with our adversaries. We shouldn’t be afraid to do so,” he said. “We’ve tried the other way. It didn’t work.”

The Illinois senator said those who questioned his statements on Pakistan were being deliberately obtuse about an action any president would take. “If we have Osama bin Laden in our sights and we’ve exhausted all other options, we should take him out before he plans to kill another 3,000 Americans. I think that’s common sense,” Mr. Obama said.

A former senator making a second bid for the Democratic nod, John Edwards of North Carolina, suggested that Mr. Obama went too far when he ruled out using nuclear weapons against Al Qaeda and that Mrs. Clinton also erred when she ruled out using such arms in Iran. “I would, as president, not talk about hypotheticals in nuclear weapons. I think that’s not a healthy thing to do,” Mr. Edwards said. “It effectively limits your options.”

Earlier this year, the Democratic hopefuls, with the possible exception of Senator Biden of Delaware, seemed to be in a competition to see who could promise the fastest withdrawal of American troops from Iraq.

That dynamic ebbed a bit yesterday, as Mrs. Clinton used some of her gloomiest language to date to describe the risks of a headlong rush to the exits in Iraq. “This is going to be very dangerous and very difficult. A lot of people don’t like to hear that,” she said. “If you look at how we would have to take our troops out, plus the equipment, which we would not want to leave, plus what we do with the people in the Green Zone, plus what we do with the Iraqis who sided with us — thousands of them — plus what we do with the more than 100,000 American contractors who are there, this is a massive, complicated undertaking.”

Mrs. Clinton rejected suggestions by another candidate, Governor Richardson of New Mexico, that the troops could be pulled out within six or eight months, in part by land through Turkey. “It is so important that we not oversell this,” she said. “There has been no indication that the Turks are willing to let us move out. They wouldn’t let us move in. That means we go back down through the south. And if you remember, when we were supposedly on the road to liberation, we were attacked by Shiites back in March and April of 2003. So this is not going to be easy or safe.”

With no particular prompting, Mrs. Clinton took a veiled swing at Mr. Obama yesterday by boasting about her vote against the Class Action Fairness Act. She called the measure, which became law in 2005 and directs many collective lawsuits to federal court, “just really another way of lining the pockets of big business.”

Mr. Obama was one of 18 Senate Democrats to vote for the bill, as was Mr. Dodd.

The candidates also opined briefly on merit pay for teachers, a practice opposed by most teachers’ unions. Messrs. Obama and Biden spoke up in favor of the idea, as did a former senator from Alaska, Mike Gravel. Mrs. Clinton said she supported bonuses based on “school-wide performance.” Mr. Dodd called performance-based teacher pay “a huge mistake.”

Most national polls show Mrs. Clinton leading by a substantial margin, followed by Mr. Obama in second and Mr. Edwards in third. The race looks tighter in Iowa, where the top candidates have traded the lead in recent surveys.

In a sign of debate fatigue among the leading contenders, Mr. Obama’s camp announced that he will limit his attendance at debates because a slew of invitations threatened to overwhelm his schedule.

“Unfortunately, we simply cannot run the kind of campaign we want and need to, engaging with voters in the early states and February 5 states, if our schedule is dictated by dozens of forums and debates,” the senator’s campaign manager, David Plouffe, wrote. He said that between now and December 10, Mr. Obama would attend only five Democratic Party-sanctioned debates, two Iowa debates, and an exchange sponsored by a Spanish-language broadcaster, Univision.

A spokesman for Mrs. Clinton, Howard Wolfson, said yesterday that the senator plans to attend at least seven of those sessions and “as many forums as scheduling permits.”