Outrage over a strike by the Israeli military that killed seven aid workers in Gaza has supercharged resistance among congressional Democrats to sending arms and fresh military funding to Israel.

The mounting concern has added uncertainty to a pending foreign aid package for Ukraine and Israel that has been stalled in the House for months. It has also fueled calls by Democrats for the administration to stop sending Israel offensive weapons already in the pipeline, some of them for many years.

Until recently, the $14.1 billion President Biden requested in the fall for Israel’s war against Hamas was regarded as a popular and bipartisan sweetener to a broader spending package that includes $60 billion in military aid for Ukraine, which faces stiff resistance from many House Republicans. But that dynamic appears to have shifted substantially in recent days, particularly after the killing on Monday night of aid workers for the anti-hunger organization World Central Kitchen.

Forty House Democrats including Representative Nancy Pelosi, the former speaker, sent a letter on Friday to Mr. Biden and Secretary of State Antony J. Blinken expressing displeasure with their approach to Israel. The group called on the Biden administration to deny Israel weapons until the completion of an inquiry into how the strike happened and tie any new aid to conditions “to ensure it is used in compliance with U.S. and international law.”

“In light of this incident, we strongly urge you to reconsider your recent decision to authorize the transfer of a new arms package to Israel, and to withhold this and any future offensive arms transfers until a full investigation into the airstrike is completed,” they wrote.

The letter, led by Representatives Mark Pocan of Wisconsin, Jim McGovern of Massachusetts and Jan Schakowsky of Illinois, was one of the largest rebukes of Mr. Biden’s approach to Israel by his allies in Congress since Israel declared war in response to the Oct. 7 attack by Hamas.

“We want to see the president be more aggressive in protecting the assistance and trying to stop the hostilities,” Mr. Pocan said in an interview on Friday.

There is broad bipartisan support in Congress for aiding Israel, and that is unlikely to change even amid widespread discontent among Democrats about the conduct of the war. But the mounting frustration could further bog down the already stalled security spending package, which the Senate passed in February but is frozen in the House amid Republican opposition to the Ukraine funding.

Speaker Mike Johnson has said he plans to bring up the security package in the coming weeks, and he would need substantial Democratic support to push it through the House.

A growing chorus of Democratic lawmakers, mostly from the party’s progressive wing, has become impatient with the president and repeatedly pushed him to leverage American weapons sales to pressure Israel to better protect civilians and guarantee that more aid will make its way to displaced Palestinians in the region.

“The U.S. has a responsibility to stop financing the Netanyahu government’s strategy, which has so disproportionately killed civilians, aid workers and medical personnel,” Senator Peter Welch, Democrat of Vermont, said in a statement on Thursday, referring to Prime Minister Benjamin Netanyahu of Israel. He echoed his many calls to restrict offensive weapons transfers to Israel.

For weeks, the group of progressives calling on Mr. Biden to take more concrete actions against America’s closest ally in the Middle East has been small but vocal. Representative Rashida Tlaib of Michigan, the only Palestinian American member of Congress, has repeatedly pressed her colleagues to join her vocal opposition to the administration’s policy in Israel and Gaza.

Last month, she and Representative Cori Bush of Missouri, another left-wing Democrat, sent a letter to the president urging him to end “any additional transfer of funds, weapons, military equipment, and any other material support.”

Friday’s letter, signed by both Ms. Tlaib and Ms. Bush, highlights how frustration on Capitol Hill has begun to spread beyond the far-left flank of the party. Senator Chris Coons of Delaware, a longtime supporter of Israel and one of Mr. Biden’s closest Democratic allies in Congress, said on Thursday that he would vote to place conditions on aid to Israel if Mr. Netanyahu conducted a major offensive in Rafah without making provisions for civilians or humanitarian aid.

“I could accept, you know, ongoing targeted attacks, special forces teams, small counterterrorism, targeted raids into Rafah,” he said in an interview. “I’m just trying to deter massive civilian casualties.”

He told CNN in a separate interview that even contemplating conditions on aid left him in new territory: “I’ve never been here before.”

Mr. Pocan said that while Congress has been on a two-week recess, lawmakers holding events in their home districts have encountered confusion and dismay among their constituents about the war and U.S. policy toward Israel.

“How can you provide both assistance and weapons at the same time to the same area?” Mr. Pocan said. He warned that the growing frustration could not only imperil a future funding package but also threaten Mr. Biden’s re-election chances.

“It’s imperative that the White House hear what we’re hearing in places like Wisconsin and Michigan and other swing states, because this is what’s on the ground and, you know, every day we’re closer to November,” Mr. Pocan said.

During a call with Mr. Netanyahu on Thursday, Mr. Biden threatened to condition future support for Israel on how it addresses concerns about civilian casualties and the humanitarian situation. The same day, Israeli government officials announced the opening of additional aid routes between Israel and northern Gaza. It was unclear how soon these routes would open.

Also on Thursday, Mr. Netanyahu used a visit to Jerusalem by 15 House Republicans to lobby for quick approval of the emergency military aid package for Israel.

“Give us the tools faster and we’ll finish the job faster,” Mr. Netanyahu told the group, whose visit was organized by AIPAC, according to a statement from the prime minister’s office. “I hope you find a way to give it as fast as you can.”

Shortly after the meeting, Mr. Johnson, who was not on the trip, said on social media that “Biden should not undercut our ally amidst an existential threat by conditioning our support.”