Senator Clinton is adding fuel to the firestorm about Senator Obama’s links to his former pastor and to a member of the Weather Underground, suggesting last night that they would present easy targets for Republicans to exploit in the fall.

In a 90-minute debate in Philadelphia, Mrs. Clinton carefully but assertively sought to fan the flames that have nipped at Mr. Obama for weeks, citing her work representing New York to take fresh offense at statements by his former pastor, the Reverend Jeremiah Wright, blaming America for the terrorist attacks of September 11, 2001. She also questioned his ties to a former member of the radical Vietnam-era group the Weather Underground,

William Ayers, who was quoted on September 11 as defending the group’s bombing of the Pentagon and the Capitol in the 1970s.

“It is clear that, as leaders, we have a choice who we associate with and who we apparently give some kind of seal of approval to,” Mrs. Clinton said, after earlier pointing to what she called Mr. Wright’s “intolerable” comments after the terrorist attacks, when he said: “America’s chickens are coming home to roost.” She added: “You know, these are problems. And they raise questions in people’s minds.”

Last night’s debate on ABC was the first for Mrs. Clinton and Mr. Obama in more than seven weeks, and it was the only remaining one-on-one event to which both candidates have accepted invitations. It comes less than a week before the April 22 primary in Pennsylvania, where most observers believe Mrs. Clinton must win to continue her campaign.

After an extended discussion of the controversies surrounding Mr. Obama, the debate turned to policy, where the Illinois senator sought to balance his commitment to prevent a nuclear Iran with his pledge to meet personally with its radical leader, Mahmoud Ahmadinejad. “I will take no options off the table when it comes to preventing them from using nuclear weapons or obtaining nuclear weapons,” he said.

Mr. Obama said he would make clear to Iran that “an attack on Israel is an attack on one of America’s strongest allies.” He later praised the first President Bush in comparison to his son, saying, “When you look back at George H.W. Bush’s foreign policy, it was a wise foreign policy.”

Mrs. Clinton used even stronger language with regard to Iran, saying an attack on Israel would incur “massive retaliation” from America if she were president.

In warning of Mr. Obama’s links to Messrs. Wright and Ayers, Mrs. Clinton pointedly did not say Mr. Obama was unelectable. Asked if he could defeat Senator McCain in November, she replied: “Yes, yes, yes.”

She said she had the advantage because her many years in public life had allowed her to be vetted over and over again. “I have a lot of baggage, and everybody has rummaged through it for years,” Mrs. Clinton said.

Leading in delegates, Mr. Obama found himself on the defensive for much of the first half of the debate at the National Constitution Center, bristling at the sustained focus on his associations with shadowy figures as a distraction from bread-and-butter policy issues.

While he largely kept an even tone, he struck back at Mrs. Clinton on several occasions. When she noted that he had served on a nonprofit board with Mr. Ayers in Chicago, he pointed out that it was President Clinton who commuted the sentences of two members of the Weather Underground during his eight years in office, which, he said with a hint of sarcasm, “I think is a slightly more significant act.”

“By Senator Clinton’s own vetting standards, I don’t think she would make it,” he said.

Mr. Obama noted that a former pastor of Mrs. Clinton’s had expressed sympathy for Mr. Wright. His campaign later confirmed that he was referring to the pastor of First United Methodist Church of Little Rock, Edward Matthews, whose comments were reported in The New York Sun.

When Mrs. Clinton repeated her criticism of Mr. Obama’s statement at a San Francisco fund-raiser that small-town Americans were “bitter” and “cling” to guns and religion, he raised her infamous comments during her husband’s first president campaign in 1992, when she offended some women by saying, “I suppose I could have stayed home and baked cookies and had teas.”

Mr. Obama recalled feeling sympathy for her then because he knew her comments were being misunderstood. “I’m sure that that’s how she felt, as well,” he said. “But the problem is that that’s the kind of politics that we’ve been accustomed to. And I think Senator Clinton learned the wrong lesson from it because she’s adopting the same tactics.”

On taxes, Mr. Obama said he would consider raising the capital gains rate to as much as 28%, while Mrs. Clinton said she would not increase it beyond 20%. It is currently no higher than 15%.

Mr. Obama said raising the tax was a way to target the windfalls of some on Wall Street. “I would look at raising the capital gains tax for purposes of fairness,” he said. “The top 50 hedge fund managers made $29 billion last year.”

The senator from Illinois rebuffed a moderator’s suggestion that cutting capital gains rates actually increases revenue. “That might happen or it might not. It depends on what’s happening on Wall Street and how business is going,” Mr. Obama said.

Mrs. Clinton said she was “certainly against” Mr. Obama’s suggestion to raise the current $102,000 cap on income subject to Social Security taxes.

She said that would hurt “educators here in the Philadelphia area or the suburbs, police officers, firefighters and the like.”

“Most firefighters, most teachers, they’re not making over $100,000 a year,” Mr. Obama replied, adding that he might exempt those with incomes just over the cap.