The party’s traditional support for Israel has been tested in recent years by a vocal liberal wing, but the attacks in Israel could allow it to bridge the divide.

The crisis in Israel is offering an opportunity for Democrats to bridge a foreign policy divide that Republicans have exploited for years and to unify their party behind a president facing one of the biggest geopolitical challenges in the region in decades.

In recent years, the Democratic Party’s traditional support for Israel has been tested by a vocal liberal wing, which has called for limiting American military and financial support. But as the full scale of the atrocities of Hamas have become clear, those voices have been largely constrained to the fringes of party politics.

Comments from more liberal members — including Representatives Ayanna Pressley of Massachusetts and Cori Bush of Missouri — calling for “an immediate cease-fire and de-escalation” within hours of the initial attack by Hamas on civilians were widely condemned by the party. Even the White House joined in, with Karine Jean-Pierre, President Biden’s press secretary, describing comments calling for a cease-fire as “repugnant” and “disgraceful.”

But that pro-Israel consensus — and Mr. Biden’s ability to rally his own party around financial and military support — could be sorely tested as Israel’s counterattack leads to greater Palestinian casualties and more images of neighborhoods in Gaza reduced to rubble.

“What is critical for us is to not lapse into moral equivocation,” said Representative Jake Auchincloss, a Democrat and Jewish military veteran who represents suburban Boston. “We have hard weeks ahead, and Congress is going to have to steel ourselves.”

Republicans have tried to capitalize on the cracks in the party. At the Capitol on Tuesday, a Fox News reporter followed Representative Rashida Tlaib, a Democrat from Michigan, pushing the first Palestinian-American woman in Congress to comment on Hamas atrocities. Ms. Tlaib refused to acknowledge the question, as she walked briskly through the halls. A day after the attack, Ms. Tlaib had described Israel as an “apartheid government” and called to stop “unconditional” American aid to the country.

Asked about the moment in an interview on Fox News the next morning, Nikki Haley, the former ambassador to the United Nations who is running for the Republican presidential nomination, said: “I don’t know how they justify 1,200 dead. I don’t know how they justify the torture. That’s between them and God.”

Ms. Tlaib later sent a statement clarifying her position. “I do not support the targeting and killing of civilians, whether in Israel or Palestine,” she said. “The fact that some have suggested otherwise is offensive and rooted in bigoted assumptions about my faith and ethnicity.”

Former President Donald J. Trump has spent years highlighting the comments of Ms. Tlaib and others to try to break the traditionally overwhelming support of Jews for Democratic candidates. In 2019, Mr. Trump even went so far as to call Jews who voted for Democrats “disloyal to Israel” — a comment that was criticized for echoing an anti-Semitic trope about dual loyalties. A 2021 survey by the American Jewish Committee, a nonpartisan group that conducts public opinion research about the Jewish community, found that 68 percent of Jewish voters reported casting their ballots for Mr. Biden in the 2020 election.

After the Hamas attacks, Democratic leaders, strategists and donors dismissed the views of lawmakers such as Ms. Tlaib as coming from a small, largely powerless faction of their party. They pointed to the long records of Mr. Biden; the Senate majority leader, Chuck Schumer of New York; and the House Democratic leader, Representative Hakeem Jeffries of New York, in backing Israel. In his remarks on Tuesday, Mr. Biden said that Israel and the United States were stronger and more secure when acting “according to the rule of law,” flicking at some of the concerns from his left flank.

“This very small group in the Democratic Party is very loud, but we shouldn’t forget that they are still a small minority,” said Haim Saban, an Israeli-American media investor who is one of the party’s top donors. “The leadership of the Democratic Party is in the place where it is in our national interests to be, which is in support of Israel.”

And yet, there are already signs that the broad consensus may be somewhat fragile. During the 2022 midterms, divides over Israel exploded in the party primaries, with more hawkish organizations targeting Democratic candidates they viewed as not supportive enough of Israel. In Michigan, the American Israel Political Affairs Committee and the Democratic Majority for Israel PAC spent millions to unseat Representative Andy Levin, who is Jewish, largely because he frequently criticized the Israeli government’s treatment of Palestinians.

Mark Mellman, the founder and president of Democratic Majority for Israel, said he had been struck by the unanimity of his party in the days after the brutal attacks.

“You have people that last week would be considered Israel critics standing up at rallies with the pro-Israel community condemning Hamas,” Mr. Mellman said. “Even Representative Alexandria Ocasio-Cortez is condemning Hamas.”

Democratic coalition have expressed more willingness to question the traditional alliance between Israel and the United States. Polling conducted by Gallup in March found that Democratic affinity for Palestinians had risen by 11 points over the past year, with a greater number of the party’s voters saying they sympathize more with Palestinians than Israelis.

Overall, however, support for Israel remains strong in the United States, with nearly seven in 10 Americans saying they feel very or mostly favorable toward Israel. By contrast, just one-quarter of Americans say they feel very or mostly favorable toward the Palestinian Authority.

Still, pro-Palestinian sentiments have roiled college campuses, traditionally a reliable source of Democratic votes in election years. At Harvard, a letter by a student coalition holding “the Israeli regime entirely responsible for all unfolding violence” has sparked a national backlash. At New York University, a law student who accused Israel of “genocide” in a student publication prompted a law firm to withdraw its job offer and the school to release multiple statements distancing itself from the comments.

“I’m worried about the next generation of policymakers,” said Mr. Auchincloss, the Democratic congressman from suburban Boston. The environment surrounding students, he said, is one that is “hostile to Zionism and increasingly hostile to Jews.”

Those divides are likely to become political fodder. In California, where a fierce race between three Democrats is underway to replace Senator Dianne Feinstein, who died last month, the issue capped an hourlong candidate forum on Sunday.

Representative Barbara Lee said the United States should call for a cease-fire. Representative Katie Porter focused on the impact in the United States, saying: “It is important to remember, as we stand with Israel, as we stand against terror, as we mourn, that we learn the lessons of our own 9/11, which gave rise to hateful Muslim-phobia and civil rights violations.”

Representative Adam Schiff, who represents a part of Los Angeles with a significant Jewish population, later criticized his opponents for not voicing steadfast support for Israel.

“We should remember what it was like for ourselves on 9/11,” he said in an interview. “And what Israel needs right now, in addition to our military support and our intelligence support, is our unequivocal moral support. I’m deeply disturbed by some of the comments of my colleagues on both sides of this war. That should have no place in our discussion of Israel right now.”

Ruth Igielnik contributed reporting from Washington.