US Senator Bernie Sanders introduced a resolution Thursday aimed at halting a $735 million arms sale to Israel, following similar moves by progressive Democrats but with very little chance of success.
A clear majority of Republicans back Israel in its military confrontation with Gaza that authorities say has claimed more than 200 lives.
Democrats are more divided, leaving President Joe Biden's administration under pressure from his party's liberal flank.
"At a moment when US-made bombs are devastating Gaza, and killing women and children, we cannot simply let another huge arms sale go through without even a Congressional debate," said Sanders, an independent.
Sanders has the power to bring the resolution to a vote even without the agreement of Senate majority leader Chuck Schumer. But it appears unlikely that the resolution would win the 51 votes necessary to pass, since many Democrats do not support such a measure.
Congress was formally informed on May 5 of the latest sale to Israel's defense ministry, totaling $735 million.
According to congressional rules, lawmakers have until Thursday to present and approve a resolution opposing the sale if it is to be blocked.
But the resolution's backers say a vote could be held even after Thursday.
On Wednesday, progressive congresswoman Alexandria Ocasio-Cortez introduced a similar resolution in the House of Representatives.
"At a time when so many, including President Biden, support a ceasefire, we should not be sending 'direct attack' weaponry to Prime Minister (Benjamin) Netanyahu to prolong this violence," she said in a statement.
Despite the Democratic majority in the House, Ocasio-Cortez's resolution also has very little chance of passing. Democratic leaders, who do not support the resolution, have not set a date for a vote.
A substantial majority of American Jews identify as Democrats, and the party traditionally has supported Israel, albeit with a few critical voices.
The latest Middle East bloodshed, however, has stirred up fresh criticism of the Jewish state among moderate Democrats.
Biden toughened his tone Wednesday, telling Netanyahu that he expects "significant de-escalation" in Israel's military confrontation with the Palestinians.
The violence between Israel and armed Palestinian groups, the worst in years, has claimed at least 239 lives -- mostly Palestinians -- in 10 days.