The debate, to be held at the National Constitution Center, is being telecast nationally by ABC and will be seen locally on 6ABC, starting at 8 p.m.

It's the first such encounter between the two Democratic candidates in nearly two months and is likely to draw the largest audience of the political season.

New polls suggested that the recent controversy may have blunted whatever momentum Obama had generated in his quest to overtake Clinton in Pennsylvania.

The surveys showed no dramatic drop in Obama's standing; in most, he remained within single digits of Clinton. But he had been gaining ground before last week.

Neither candidate was in the Philadelphia area yesterday. But their spouses were, with former President Bill Clinton campaigning in Coatesville, Phoenixville and Quakertown, and Michelle Obama speaking at Haverford College.

Tonight's debate will be far from the only time the two candidates appear on area televisions between now and Tuesday's primary. The two campaigns have bought at least $4.5 million of time for commercials in the closing week.

According to industry sources, Obama has bought more than $3 million of time, breaking the Pennsylvania record he set last week. About 60 percent of the money is going for ads in the Philadelphia market.

Clinton has purchased at least $1.4 million, her largest buy to date in Pennsylvania.

Much has happened since they last debated in Ohio on Feb. 26, including furors over the comments of Obama's former pastor; Clinton's repeated misrepresentation of her 1996 landing in Bosnia while first lady; and Obama's recent comments about the "bitter" residents of small towns.

To what degree those topics dominate the debate will depend both on the nature of the questions, to be posed by Charles Gibson and George Stephanopoulos of ABC News, and on the inclination of the candidates.

Howard Wolfson, Clinton's communications director, said yesterday of Obama's recent statement: "I believe this is an issue of tremendous importance to Pennsylvanians."

Sen. Bob Casey, Obama's top supporter in the state, disagreed, saying on NBC's Today show: "People want to hear about the issues.. . . I think they want to hear about solutions for the economy of Pennsylvania."

Yesterday, Obama spoke to a group of veterans in Washington, Pa., where he was endorsed by state Auditor General Jack Wagner.

He condemned the Bush administration's treatment of veterans, the substandard care given wounded soldiers, and military families being "left to fend for themselves."

"We've heard rhetoric that hasn't been matched by resources," Obama said. "It's not acceptable. You cannot lead this country into war, and then fail to care for those who have served and for their families."

Recalling his grandfather's service in World War II, he pledged to have "zero tolerance" for veterans sleeping on the street.

During the question-and-answer session, an audience member suggested that Clinton's calling Obama "elitist" was close to calling him "uppity." Obama disagreed.

"I don't think there are racial overtones to the attacks going on.. . . It's politics," the Illinois senator said, adding that he hoped it was the end of the "silly season."

He reinforced the latter message with a new commercial showing Clinton getting jeered Monday in Pittsburgh as she raised the issue of his "bitter" comments.

Clinton, in a speech to newspaper editors in Washington, said her agenda for her first 100 days in office included beginning a troop withdrawal from Iraq and signing legislation President Bush vetoed - namely, embryonic stem-cell research and children's health insurance.

"In short, starting from Day One, the Bush-Cheney era will be over in name and in practice," she said.

She also said she would ask Congress to reverse some of Bush's tax cuts, would tackle global warming, and would urge Canada and Mexico to renegotiate NAFTA.

The Clinton campaign put out a new TV ad of its own, pointing out that Obama, while not taking money from lobbyists and political action committees, had received more than $200,000 from employees of oil companies.

Bill Clinton spoke for 45 minutes at a steelworkers' union hall in Coatesville, emphasizing his wife's plans for health-care coverage, energy, the economy and the environment.

"If Pennsylvania gives her a big victory, she can win," he said to cheers.

Daughter Chelsea Clinton campaigned in Waynesburg, Johnstown, Chambersburg and Mechanicsburg.

Michelle Obama sang her husband's praises for more than an hour at Haverford College, describing him as compassionate, hardworking and hopeful.

"There's something special about Barack," she told the crowd of several hundred, many of them college students nodding in agreement.

Four new statewide polls, some conducted through Monday, showed Clinton still leading in Pennsylvania.

In the Quinnipiac University poll, Clinton was ahead 50 percent to 44 percent, exactly as she had been a week ago. The Rasmussen Reports survey had Clinton in front 50 percent to 41 percent; a week earlier, she led 48 percent to 43 percent.

Clinton also led in the SurveyUSA poll, 54 percent to 40 percent. A week ago, she had 56 percent to Obama's 38 percent. And a Los Angeles Times/Bloomberg poll gave Clinton a 46 percent to 41 percent edge.

Nationally, Obama retained his leads in the two most-watched daily tracking surveys - up 11 points over Clinton in the Gallup poll, up 7 in the Rasmussen survey.

Get more coverage of the presidential campaign online at http://go.philly.com/paprimary

Inquirer staff writers Kathleen Brady Shea and Kathy Boccella contributed to this article. It also includes information from the Associated Press.