A coalition of a dozen liberal organizations and labor unions sent a letter to the White House on Thursday night demanding that President Biden end military aid to Israel until its government lifts restrictions on humanitarian aid to Gaza, the latest indicator of shifting mainstream Democratic opinion on the war.

The group includes not only progressive groups like MoveOn and the Working Families Party, but also the mainstream Democratic Center for American Progress and NextGen America, the organization founded and funded by Tom Steyer, a billionaire who ran for president in the 2020 Democratic primary. Other signatories to the letter include the Service Employees International Union and the National Education Association, labor unions that make up key elements of the Democratic Party.

The letter calls on Mr. Biden to enforce the Foreign Assistance Act, which bars military support from going to any nation that restricts the delivery of humanitarian aid.

“This will send a clear message that the Netanyahu government is not above the law and that the U.S. will not stand by while the war kills innocent Palestinians and continues to drive escalation throughout the region,” the letter states, referring to Israel’s prime minister, Benjamin Netanyahu. “U.S. law is unequivocal: Countries that obstruct U.S. humanitarian aid cannot receive U.S. military aid under the Foreign Assistance Act or the Arms Export Control Act.”

Cristina Tzintzún Ramirez, the president of NextGen America, which focuses on driving voter turnout among young people, said there was growing risk that Mr. Biden will lose support from a key part of the Democratic coalition if there is not a significant change in the American position toward the war in Gaza.

“We are concerned with the humanitarian and moral implications and the political survival of the administration,” Ms. Tzintzún Ramirez said. “We’ve seen a surge of young people say they care about foreign policy and this issue in a way we have not seen historically.”

Last week, Mr. Biden told Mr. Netanyahu that the United States could withhold support for Israel if it does not do more to protect civilians and ensure adequate supplies for Gaza. And since then the president has repeated that getting more aid into Gaza is a priority.

“They need to do more,” Mr. Biden said of Israel’s government and Mr. Netanyahu during a news conference Wednesday. “There’s one more opening that has to take place in the north. So we’ll see what he does in terms of meeting the commitments he made to me.”

The White House and the Biden campaign declined to comment on the letter.

The letter adds to the growing pressure Mr. Biden has faced from across the Democratic Party as Israel’s war in Gaza enters its seventh month. More than 33,000 people have been killed in Gaza, according to the local health authorities. About 1,200 people were killed in Israel when Hamas militants launched an attack that instigated the conflict on Oct. 7.