WASHINGTON - The U.S. Senate is expected to hold a vote on Tuesday on a resolution that aims to block a small portion of President Donald Trump's massive weapons deal with Saudi Arabia. While the resolution is likely to fail, the senators who proposed it said that they expect a close vote.

"We're going to surprise a lot of people," said Connecticut Democrat Sen. Chris Murphy, one of the co-sponsors of the resolution.

Murphy, together with Senators Al Franken, a Democrat from Minnesota, and Rand Paul, a Kentucky Republican, aren't trying to cancel the entire list of arms deals – worth billions of dollars – that the Trump administration claims it has signed with Saudi Arabia. They focus on one specific deal, in which the Saudi regime is planning to purchase half a billion dollars' worth of precision-guided munitions. The Saudis plan to use these munitions in Yemen, where a civil war has been taking place since 2015.

When talk of a Senate resolution against the Saudi deal first emerged, the assessments on Capitol Hill were that it would have almost zero chances of passing. The combined forces with an interest of seeing the deals go through – the White House, the Pentagon, the U.S. Defense industry and the Saudi government – were perceived as too powerful. In addition, the Senate would need a veto-proof majority in order to cancel the deal, which seems very unlikely right now given the Republicans' control both houses of Congress.

Yet the resolution presented by Murphy, Franken and Paul is focused solely on the munitions sale, which is a relatively small part of the entire arms deal, and seemed to be picking up steam on Monday. Senate Minority Leader Chuck Schumer announced his support for it, as did Sen. Ben Cardin of Maryland, the most senior Democrat on the Foreign Relations Committee. Cardin said that instead of selling munitions to Saudi Arabia, the Trump administration needs to present a strategy for ending the war in Yemen.

The biggest question is how many Republicans will come out against the munitions deal. Sen. Paul, who holds libertarian and anti-interventionist positions, is an outlier in the party, and most Republicans aren't expected to go head-to-head with the president over this issue. Last year, when Paul tried to block a weapons deal that the Obama administration signed with Saudi Arabia, he was rejected by the Senate by a vote of 71-27.

"This time will probably be much closer," one Democratic staffer involved in the discussions told Haaretz, "but the chances of actually blocking it are slim."