WASHINGTON — Senator Biden delivered a heartfelt endorsement of Senator Obama’s commitment to Israel here yesterday, telling an audience of Jewish Democratic activists that he would not have joined the party’s ticket if he were not convinced of Mr. Obama’s support for the Jewish state.

“My support for Israel begins in my stomach, goes to my heart, and ends up in my head,” Mr. Biden told the National Jewish Democratic Council at the group’s annual conference. “I promise you. I guarantee you, I guarantee you, I would not have joined Barack Obama’s ticket as vice president if I had any doubt, even the slightest doubt, that he shares the same commitment to Israel that I share,” he said, drawing a standing ovation from a crowd estimated at 300 people.

Mr. Biden vouched for Mr. Obama as the Democratic ticket is engaged in a fierce campaign in Florida, where the support of Jewish voters could make the difference in a close race. Polls have shown Mr. Obama leading the Republican nominee, Senator McCain, among Jewish voters, but the gap is smaller than in past elections. The Illinois senator for months has fought rumors spread through an e-mail and whisper campaign that he is a Muslim, and several council members here said the false reports about his religion — he is a Christian — have made an impact.

“The e-mails have done some damage, so they have a lot of work to do in that area,” an NJDC member from Olney, Md., Meryl Trachtman, said.

Mr. Biden made only a glancing reference to the whisper campaign during his 30-minute address, telling the council to “ignore the e-mails.”

After speaking briefly about Mr. Obama’s stance on Israel, Mr. Biden delivered his standard stump speech, criticizing Mr. McCain as a continuation of President Bush. He compared the Arizona senator’s pledge to reform Washington and improve his own party to Mr. Bush’s promise of “compassionate conservatism” eight years ago.

“We’ve seen that movie before, and we know from experience that the sequel is always worse than the original,” Mr. Biden said.

Council members said Mr. Biden, a Delaware senator since 1973, has a strong relationship with the Jewish community and a solid record on Israel. But a few were unsure if he could help Mr. Obama overcome doubts among Jewish voters. “I cannot tell you how many people who have told me, ‘He’s a Muslim,'” Roz Jonas of Bethesda, Md., said, referring to Mr. Obama. “I think maybe Biden helps that a little.”