WASHINGTON — The Israeli airstrikes that claimed the lives of seven World Central Kitchen aid workers have stunned and outraged Democrats in Congress, fueling a fresh effort by lawmakers to push President Joe Biden to impose conditions on U.S. military aid to Israel.

Frustration has been mounting for months among Democrats in Congress over how Israeli Prime Minister Benjamin Netanyahu’s government has carried out its military offensive in the Gaza Strip and the dire humanitarian conditions faced by Palestinian civilians. But Monday’s lethal strikes on a World Central Kitchen team jolted many lawmakers and their aides, who say the attack could serve as a defining moment for America’s approach to the war in Gaza and U.S. relations with Israel.

“This week could be a turning point,” one congressional official said.

Democratic Sen. Peter Welch of Vermont told NBC News that the attack on the aid convoy is “an indication that there has been no modification of the Netanyahu war plan” and “it’s creating increasing concern among more and more of my colleagues.”

Even lawmakers who initially refrained from sharply criticizing Israel over its tactics in Gaza have joined calls for a cease-fire or warned Netanyahu’s government not to enter the southern city of Rafah without a plan to safeguard the lives of its civilian population.

Democratic Sen. Chris Coons of Delaware, a close Biden ally and a longtime supporter of Israel, came out for the first time Thursday in favor of placing conditions on U.S. arms shipments to Israel if civilians are not protected in Rafah.

“If Benjamin Netanyahu were to order the IDF into Rafah at scale … and make no provision for civilians or for humanitarian aid, I would vote to condition aid to Israel,” he told CNN. “I’ve never said that before, I’ve never been here before.”

According to Jeremy Ben-Ami of J Street, a liberal lobbying group that says it is “pro-Israel and pro-peace,” more and more Democrats are urging the White House to back up its critical words with concrete actions.

“I think there are a growing number of Democratic members who would like to see the administration doing more than it’s doing,” he said.

On Friday, more than three dozen members of Congress signed a letter urging Biden and Secretary of State Antony Blinken to “reconsider” the decision to authorize an arms package transfer to Israel. The letter, notably signed by former House Speaker Nancy Pelosi, D-Calif., also calls for the U.S. to withhold future arms transfers pending an investigation into the airstrike on the World Central Kitchen humanitarian workers or if Israel “fails to sufficiently mitigate harm to innocent civilians in Gaza.”

“In light of the recent strike against aid workers and the ever-worsening humanitarian crisis, we believe it is unjustifiable to approve these weapons transfers,” the letter said.

A spokesperson for Pelosi told NBC News in a statement that she is “sympathetic to some of the thoughts in the letter, and she feels very strongly that there must be a comprehensive, independent investigation into the horrific killing of the World Central Kitchen heroes,” adding that she “knows President Biden’s support for Israel and empathy with the innocent civilians in Gaza, and she respects his judgment in how to proceed.”

A White House spokesperson did not immediately respond to a request for comment on the letter Friday evening.

Pressure from Democrats in Congress could lead the administration to possibly delay the delivery of certain offensive weapons or military assistance, congressional aides said. Although such a move would apply to hardware due to be delivered months or years from now and would not affect Israel’s current supply of arms and ammunition, it would send an unprecedented political message to Israel.

“I think a more limited and symbolic step is a real possibility,” Ben-Ami said.

Biden administration officials also have described the strikes on the aid convoy as a pivotal moment.

In a phone call with Netanyahu on Thursday, Biden strongly implied that Washington was prepared to condition military assistance to Israel over how it addresses the humanitarian crisis in Gaza and cease-fire talks, according to two U.S. officials.

After Biden’s warning, Israel announced that it would open additional aid routes to allow for increased assistance to flow into Gaza, including the Ashdod port and the Erez crossing on Israel’s border with Gaza.

On Friday, the Israeli military said that it removed two officers from their posts over the lethal strikes on the World Central Kitchen convoy and that an investigation found serious errors and violations of protocol that led the Israel Defense Forces to repeatedly hit the convoy in the belief that Hamas gunmen were present.

“The strike on the aid vehicles is a grave mistake stemming from a serious failure due to a mistaken identification, errors in decision-making, and an attack contrary to the Standard Operating Procedures,” the IDF said.

Welch praised Biden’s public comments condemning the strikes on the World Central Kitchen convoy, but said that it was time for concrete steps.

“The president gave a very strong statement, indicating his horror and dismay at what just happened with the bombing of the World Central Kitchen workers … he’s moved from quiet discussion with Prime Minister Netanyahu to public statements of disapproval,” Welch said. “The next step is explicit action.”

Democrats in Congress are now focusing on the looming deadline of May 8, when the administration will have to declare whether Israel is abiding by international humanitarian law and U.S. human rights laws. Under a national security memorandum issued by Biden in February, the State Department has to formally assess if Israel’s assurances are “credible and reliable” and report its findings to Congress. If the administration concludes that Israel is failing to live up to its assurances, the president will have the option to suspend additional U.S. arms transfers.

Mounting concerns among Democrats about the situation in Gaza also threaten to jeopardize a long-delayed security assistance package for Ukraine, Israel and Taiwan, which passed the Senate but has been held up for months in the House of Representatives.

Until now, the main stumbling block was House Speaker Mike Johnson and some other Republicans who are wary of defying former President Donald Trump’s opposition to additional aid for Ukraine. But a growing number of House Democrats are now reluctant to sign off on more military assistance for Israel, particularly after the strikes on the World Central Kitchen convoy, congressional aides and lawmakers said.

If the Biden administration remains reluctant to impose conditions on aid to Israel, the top two Democrats on the Senate and House foreign relations committees could try to ratchet up the pressure on the White House by refusing to approve a proposed $18 billion arms package for Israel, which includes more F-15 fighter jets. Sen. Ben Cardin of Maryland, chair of the Senate Foreign Relations Committee, and Rep. Gregory Meeks of New York, ranking member of the House Foreign Affairs Committee, have yet to say publicly if they will sign off on the planned sale.

The reaction on Capitol Hill to the airstrikes on the aid convoy illustrates the wider shift underway in how Israel is perceived in Washington. Unconditional bipartisan support for Israel is no longer a given, and Democrats are now prepared to level sharp criticism at America’s ally.

The shift is partly driven by changing attitudes among Democratic voters, particularly American Muslims and younger progressives, who are dismayed at the plight of Palestinian civilians and are less sympathetic to Israel’s arguments. Some Democratic lawmakers are under intense pressure from their constituents over the war in Gaza, and worry the issue could damage Biden’s re-election prospects in a razor-thin election race.

Even before the Hamas attack of Oct. 7 and Israel’s military offensive in Gaza, Biden was facing the prospect of a difficult re-election fight with the narrowest of margins determining the outcome. The war in Gaza has added yet another challenge for him and the Democrats as they brace for possible platform fights and street protests at the Democratic Party’s national convention in Chicago this summer.

“They’re now not so far out from the convention. What do you want to have the streets of Chicago look like in four months?” Ben-Ami said. “There’s a lot of politics just beneath the surface.”