The staggering Hamas surprise attack on Israel has done what Prime Minister Benjamin Netanyahu never could — united America’s fractious politicians behind his nation again. Sort of. Mostly. For now.

After years in which support for Israel had turned into an increasingly partisan issue in Washington, Democratic and Republican leaders in recent days have generally responded with broad condemnation of Hamas and expressions of solidarity following the slaughter of hundreds of Israeli civilians.

But that surface unity had already begun to fray around the edges by Monday as Israel retaliated with punishing airstrikes on Gaza, cut off food and water to the enclave and prepared for what may become a ground invasion that could further endanger Palestinian civilians. Some on the left wing of the Democratic Party were criticizing Israel for “apartheid” policies oppressing Palestinians and calling for an end to U.S. aid.

For the moment, such sentiments have been restricted to the fringes of the House Democratic caucus and have drawn swift rebukes from more centrist members of the party. The reality, however, is that the sympathy and support for Israel will be tested the longer the fighting continues and the more firepower that Israeli forces employ, according to lawmakers and political strategists. The challenge for President Biden and his allies is to translate the current outrage at Hamas into a sustained consensus for Israel.

The sensitivity of that was made clear by mixed signals sent by Mr. Biden’s own administration. The State Department’s Office of Palestinian Affairs posted a message on X, the platform formerly known as Twitter, on Saturday pressing “all sides to refrain from violence and retaliatory attacks,” and the account of Secretary of State Antony J. Blinken posted a message on Sunday encouraging Turkey’s “advocacy for a cease-fire.” Both posts were subsequently deleted following an outcry from Israel supporters who said it was too soon to urge Israel to stand down.

Both Mr. Biden and Mr. Blinken have otherwise steadfastly declared that Israel has a right to defend itself. The administration has begun shipping military equipment to Israel to replenish its supplies and moving American warships and planes to the region to deter Iran or other enemies of Israel from escalating the fight.

In a written statement on Monday, Mr. Biden implicitly compared the Hamas strike to the Sept. 11 attacks on the United States. “We remember the pain of being attacked by terrorists at home, and Americans across the country stand united against these evil acts that have once more claimed innocent American lives,” he said. “It is an outrage. And we will continue to show the world that the American people are unwavering in our resolve to oppose terrorism in all forms.”

With more than 900 Israelis reported killed and about 150 taken hostage, including women and children, administration officials agreed that Mr. Netanyahu had no choice but to respond with overwhelming force. Any country in the world would retaliate if its territory were invaded by land, sea and air and its citizens massacred at a concert or dragged out of their homes to be shot or hauled off as captives.

For Mr. Biden and the United States, the fact that at least 11 Americans were killed and an unknown number possibly taken hostage makes it an American crisis as well. On Monday, Hamas threatened to execute a civilian hostage every time an airstrike hits Gaza “in their homes without warning,” raising the stakes for Washington.

“I want you to know that in the Congress of the United States” at the moment “there is unity, bipartisan unity in support of what we need to do, whether it’s militarily, whether it’s diplomatically, whether it’s financially to help our friends, the Israelis,” Representative Nancy Pelosi of California, the former Democratic House speaker, told a forum organized by a Jewish group in San Francisco on Sunday.

Representative Ro Khanna, another California Democrat, said on Monday that the bipartisan support was real. “This is personal for many of my colleagues,” he said. “We know Israelis who have been killed and know Americans who have stayed in the homes of Israelis who were taken hostage. We are angered by the innocent Americans who were killed. There will be overwhelming support for Israel to defend against this act of terrorism much like the world was united behind America after 9/11.”

But that does not mean support is unlimited. The administration and lawmakers on both sides of the aisle broadly backed retaliation against Hamas itself, but to the extent that the response punishes the civilian population of Gaza, already an impoverished coastal enclave blockaded by Israel and Egypt, it could chip away at the support over time.

As of Monday, the Palestinian Health Ministry in Gaza reported that 687 Palestinians had been killed, including 140 children. Israel’s defense minister ordered a “complete siege” of the Gaza Strip, cutting off electricity, food, water and fuel for its two million residents, and said his country was at war with “human animals.”

Advocates for Palestinians lamented the largely uniform pro-Israel response in Washington. “There are few things that require less political courage in Washington than condemning Palestinians, sending Israel bombs and then turning your back on dealing with the root causes of oppression that drives political violence threatening the lives of Palestinians and Israelis alike,” said Yousef Munayyer, head of the Palestine/Israel Program at the Arab Center Washington D.C.

“People who genuinely care about peace and justice for all people see through this,” he added. “Unfortunately, there are a lack of them in this town.”

The Israeli-Palestinian conflict has been a flashpoint in Washington for years, but since the days of President Barack Obama’s administration, the once-traditional bipartisan consensus has evolved into a much more partisan divide.

Republicans have made support for Israel an inviolable litmus test issue. While in office, President Donald J. Trump tilted strongly toward Mr. Netanyahu’s government, recognizing Jerusalem as the Israeli capital and the Golan Heights as Israeli territory while cutting aid to Palestinians and forcing their office to close in Washington. Democrats, for their part, have grown more split, with some still solidly backing Israel while others have been more critical of settlements, occupation and plans to annex territory.

Mr. Biden is a case study. For decades as a senator, he positioned himself as a strong supporter of Israel, but his relationship with Mr. Netanyahu has been fairly frosty at times. Among other things, they have been at odds over the prime minister’s efforts to curb the power of the courts in his country and the American efforts to negotiate a nuclear agreement with Iran.

Mr. Biden only last month met with Mr. Netanyahu for the first time since the prime minister returned to office in December, and even then on the sidelines of the U.N. General Assembly, not at the White House. But the two appeared eager to put their differences to the side, and the president indicated that he would host the prime minister at the White House by the end of the year.

The president has offered nothing but unflinching support for Israelis since Saturday’s explosion of violence. In his statement on Monday, he said that “the American people stand shoulder to shoulder with Israelis” and that the two nations are “inseparable partners” in the fight against terrorism.

The voices of dissent among Democrats came from representatives like Rashida Tlaib, a Palestinian American from Michigan, and Cori Bush, a liberal from Missouri. Ms. Tlaib wrote on Instagram that “I grieve the Palestinian and Israeli lives lost” and added that the path to a just future “must include lifting the blockade, ending the occupation and dismantling the apartheid system that creates the suffocating, dehumanizing conditions that can lead to resistance.”

Ms. Bush likewise mourned the loss of life on both sides and said in a statement posted on X that she would “strongly condemn the targeting of civilians.” But she called for “ending U.S. government support for Israeli military occupation and apartheid.”

Other Democratic representatives, though, took umbrage. “I have zero tolerance for their uninformed responses,” Representative Josh Gottheimer, a Jewish Democrat from New Jersey, said in an interview. He called such comments sickening.

While he acknowledged that the duration and intensity of the Israeli response might have an effect, he added that the scale and scope of the Hamas attack made the calculus different from that of other eruptions between Israelis and Palestinians. “It involves Americans,” he said. “It puts it in a whole other light. I don’t think there will be much sympathy for those weak naysayers who are not focused on protecting our most important ally in the region and standing up to terrorists.”