PHILADELPHIA (AP) — Hillary Rodham Clinton said emphatically Wednesday night that Barack Obama can win the White House this fall, undercutting her efforts to deny him the Democratic presidential nomination by suggesting he would lead the party to defeat.

“Yes, yes, yes,” she said when pressed about Obama’s electability during a campaign debate six days before the Pennsylvania primary.

Asked a similar question about Clinton, Obama said “Absolutely and I’ve said so before” — a not-so-subtle dig at his rival who had previously declined to make a similar statement about him.

In a 90-minute debate, both rivals pledged not to raise taxes on individuals making less than $200,000, and said they would respond forcefully if Iran obtains nuclear weapons and uses them against Israel.

“An attack on Israel would incur massive retaliation by the United States,” said Clinton.

Obama said, “The U.S. would take appropriate action.”

They differed over Social Security when Obama said he favored raising payroll taxes on higher-income individuals. Clinton said she was opposed, her rival quickly cut in and countered that she had said earlier in the campaign she was open to the idea.

Under current law, workers must pay the payroll tax on their first $102,000 in wages. Obama generally has expressed support for a plan to reimpose the tax beginning at a level of $200,000 or more.

The debate was the 21st of the campaign for the nomination, an epic struggle that could last weeks or even months longer.

Pennsylvania, with 158 delegates at stake, is a must-win contest for Clinton, who leads in the polls and hopes for a strong victory to propel her through the other states that vote before the primary season ends on June 3.

Obama leads in the delegate chase, 1,643-1,504, with 2,025 needed for the nomination. And despite a recent gaffe, he picked up endorsements during the day from three superdelegates from a pair of states with primaries on May 6 — Reps. Andre Carson of Indiana and Mel Watt and David Price of North Carolina.

After primaries and caucuses in 42 of the 50 states, Obama leads his rival in convention delegates, popular votes and states won. She is struggling to stop his drive on the nomination by appealing to party leaders who will attend the convention as superdelegates that he will preside over an electoral defeat at a moment of great opportunity after eight years of Republican rule.

The former first lady has never denied published reports that she once told New Mexico Gov. Bill Richardson that Obama couldn’t win when he called to tell her he would be endorsing the Illinois senator.

And at a news conference earlier this month in California, Clinton sidestepped when asked directly whether Obama would win if he were the Democratic nominee. “I am sure we will have a united Democratic Party. I will do everything possible to make sure we can win and I am confident we will have a Democrat in the White House next year,” she said at the time.

Asked a similar question at the debate, she provided a similar answer at first. “I think we have to beat John McCain and I have every reason to believe we’re going to have a Democratic president and it’s going to be Barack or me.”