Former Vice President Joseph R. Biden Jr. told donors on a call on Tuesday that it was important to condemn criticism of Israel that drifts toward anti-Semitism, including on the political left, even as he acknowledged that he had “gotten in trouble” for such calls in the past.

“Criticism of Israel’s policy is not anti-Semitism,” Mr. Biden said. “But too often that criticism from the left morphs into anti-Semitism.”

Mr. Biden, who was answering a question about anti-Semitism on the left in the United States and England, added that “arguably, we haven’t heard enough about the Holocaust because people are still trying to deny its horrible reality.”

“So many people forget,” he went on, “it’s almost hard to believe.”

The remarks came as Mr. Biden works to unite the Democratic Party and appeal to progressives, especially younger ones, who viewed him with suspicion during the primary race. A cohort of younger progressives have been outspoken in opposition to Israel’s policies toward Palestinians, particularly under Prime Minister Benjamin Netanyahu. Mr. Biden did not cite any specific examples of anti-Semitic comments on the left or specifically identify individuals or groups that he was concerned about.

Mr. Biden himself said he was disappointed in Mr. Netanyahu for having moved “so, so far to the right” and called for Israel to “stop the threat of annexation” of West Bank territories.

“It’ll choke off any hope of peace,” Mr. Biden said, according to a pool report.

Mr. Biden said that “my commitment to Israel is absolutely unshakable” and pledged to reverse actions taken under President Trump, including restoring diplomatic relationships with the Palestinian Authority. Mr. Biden has previously said that Mr. Trump should not have moved the United States Embassy to Jerusalem from Tel Aviv but that he would not move it back.

The former vice president was speaking at a virtual fund-raiser hosted by Dan Shapiro, a former ambassador to Israel, and Deborah Lipstadt, a professor of Jewish history and Holocaust studies at Emory University. The Biden campaign said 550 people attended.

Mr. Biden was asked directly by one of the hosts about how to respond to anti-Semitism on the left, according to a transcript provided by a person on the call. “We have to condemn it, and I’ve gotten in trouble for doing that,” the former vice president replied. “Whatever the source, right, left or center.”

On American college campuses and beyond, efforts to boycott Israel as part of what is known as the Boycott, Divestment and Sanctions movement have gained adherents and opponents. Both Democratic and Republican leaders have been critical of the movement, and the House overwhelmingly passed a condemnation of it last year.

A day before the fund-raiser, Mr. Biden had written a letter to a group, Democratic Majority for Israel, that opposed his leading rival, Senator Bernie Sanders, during the primaries. “I am grateful for and humbled by your statement of support for this campaign,” Mr. Biden wrote. The letter was read aloud during an online meeting by Antony J. Blinken, a senior Biden adviser.