WASHINGTON (AP) — In a notable test Tuesday, Sen. Bernie Sanders is forcing colleagues to vote on record whether to investigate human rights abuses in the Israel-Hamas war, a step toward potentially limiting U.S. military aid to Israel as its devastating attacks on Gaza grind past 100 days.

The Senate vote, a first of its kind tapping into a decades-old law, would require the U.S. State Department to, within 30 days, produce a report on whether the Israeli war effort in Gaza is violating human rights and international accords. If so, U.S. military aid to Israel, long assured without question, could be quickly halted.

While the Senate is unlikely to approve the measure, the vote by senators will begin to reveal the depth of unease among U.S. lawmakers over Israel’s prosecution of the war against Hamas. With no apparent end to the bombardment, Israel’s attacks against Palestinians, an attempt to root out Hamas leaders, are viewed by some as disproportional to the initial terrorist attack on Israel.

The Biden administration, with repeated overtures to Israel Prime Minister Benjamin Netanyahu’s government, including shuttle diplomacy last week by U.S. Secretary of State Antony Blinken, is pushing Israel to shift the intensity of the battle. Some 24,000 people in Gaza, the majority of them women and children, have been killed and the bombings have destroyed most of the housing units, displacing most of its 2.3 million people in a humanitarian catastrophe.

“To my mind, Israel has the absolute right to defend itself from Hamas’ barbaric terrorist attack on October 7, no question about that,” Sanders told AP during an interview Monday ahead of the vote.

“But what Israel does not have a right to do — using military assistance from the United States — does not have the right to go to war against the entire Palestinian people,” said Sanders, the independent from Vermont. “And in my view, that’s what has been happening.”

Heading toward the vote, Sanders said senators are nervous because what he’s trying to do is unprecedented in procedure and essentially practice.

“The Congress has always been supportive of Israel in general, and this begins to question the nature of the military campaign,” Sanders said. “And I think that makes some other people quite nervous.”

The White House has rejected approach from Sanders as “unworkable” as it seeks a transition from Israel and works to ensure support at home and abroad against a stirring backlash to the scenes of destruction from Gaza.

“We do not believe that this resolution is the right vehicle to address these issues. And we don’t think now is the right time. It’s unworkable, quite frankly,” said a statement from the White House National Security Council’s John Kirby.

“The Israelis have indicated they are preparing to transition their operations to a much lower intensity. And we believe that transition will be helpful both in terms of reducing civilian casualties, as well as increasing humanitarian assistance,” Kirby said.

The action comes as Biden’s request for $106 billion supplemental national security aid for Israel as well as Ukraine and other military needs is at a standstill. Republicans in Congress are insisting on attaching vast policy changes to stop the flow of immigration at the U.S.-Mexico border.

Of that supplemental aid package, more than $14 billion would go to Israel, including $10 billion in U.S. military assistance, as it retaliates against Hamas for the Oct. 7 surprise attack, among the most deadly assaults ever. Some 1,200 people were killed and 250 taken hostage, many still being held.

Several key Democratic senators have announced their unease with Israel’s war in Gaza, insisting the Biden administration must do more to push the Netanyahu government to reduce civilian casualties and improve living conditions for Palestinians in Gaza.

Going further, Sanders had already announced his refusal to support more military aid to Israel in the package because of the war.

“If I had my druthers, that’s what I would do. That is not what this resolution is about,” Sanders said.

But he did say that the resolution should be seen as “a first step, not a last step.”

The resolution is drawn from the Foreign Assistance Act of 1961, which was amended after the Nixon era, enabling Congress to provide oversight of U.S. military assistance abroad. It requires that any arms or military aid must be used in accordance with international human rights accords.

While senators have voted to try to halt foreign arms sales to other countries in the past, this is an untested mechanism.

The question before the Senate will be whether to ask the State Department for a report on whether human rights violations using U.S. equipment may have occurred during Israel’s current campaign against Gaza, according to Sanders’ office.

If the resolution were to be approved, it would force the State Department to produce a report of its findings within 30 days or risk the aid being cut off.

While it’s not at all certain that U.S. aid to Israel would actually be halted, since Congress could take steps to ensure no interruption, it is enough of a threat that many senators, even the Democrats who have raised concerns about the bombardment of Gaza and the humanitarian crisis, will be unwilling to support the measure.

Republican senators are likely to fully reject Sanders’ proposal. Senate Republicans have been almost unanimous in their support for Israel, even as they are blocking Biden’s broader national security package because of divisions within the GOP over helping Ukraine as it battles Russia’s invasion.

Talks on attaching the U.S-Mexico border security provisions to the national security aid package are lumbering along, but no quick breakthrough is expected as Republicans push for tougher restrictions on migrants than Democrats are willing to give, particularly for immigrations seeking asylum in the U.S.

Associated Press writer Ellen Knickmeyer contributed to this report.