While President Trump briefly greeted Saudi Crown Prince Mohammed bin Salman in Argentina over the weekend, his allies on the Hill were feverishly working behind the scenes to stifle a Senate revolt on Yemen before it embarrasses the president further.

Last week, 63 senators dealt the Trump administration a stunning rebuke by advancing a bill to pull all American support from the Saudi-led coalition’s disastrous intervention in Yemen’s civil war. But the anger behind the vote is being fueled by the administration's withholding of U.S. intelligence on the brutal slaughter of U.S. resident Jamal Khashoggi at the hands of top Saudi security officials in early October.

Many Republicans normally on Trump's side weren't shy about their motives: “I’m pissed,” Sen. Lindsey Graham (R-S.C.), who joined 13 other Republicans in supporting the measure. “The way the administration has handled the [Khashoggi killing] is just not acceptable.”

Now, as the bill makes it way to the Senate floor for debate as soon as Wednesday, Congress faces a crucial test: Is the Yemen vote merely about scaring Trump, or is Congress going to reassert itself in their constitutionally-mandated role in overseeing foreign wars?

The CIA concluded the Khashoggi killing was directed by the crown prince himself, who the Wall Street Journal is now reporting communicated directly with the ringleader of Khashoggi’s assassination on the day he’s believed to have been dismembered. Trump has repeatedly dismissed the CIA’s findings and has repeatedly said that even if they are true, Saudi arms sales outweigh the findings of the intelligence community.

Yemen showdown

So far the Trump administration has fumbled numerous attempts to explain its inaction on the Khashoggi killing. Last week they tried sending Secretary of State Mike Pompeo and Defense Secretary James Mattis to the Capitol. But the briefing backfired spectacularly, in part because CIA Director Gina Haspel wasn’t there, as Senate leaders had demanded.

Sen. Chris Murphy (D-Conn.) has been lobbying for three years for a resolution that would tie Trump’s hands in Yemen, but it has always gone down in defeat. But this time, Murphy doesn’t think the administration can placate the growing number of senators merely by sending the CIA director to brief them on the ghoulish details of Khashoggi’s death, as Sen. Graham is demanding.

“It wasn’t just that Haspel wasn’t there; it was that they had no answers for what their policy on Yemen was, and I think everyone found the briefing very dissatisfying,” Murphy told reporters at the Capitol after the vote.

Murphy is optimistic that momentum's building for his side, which includes the bills from other main sponsors Sens. Bernie Sanders (I-Vt.) and Mike Lee (R-Utah). He says last week’s vote increases pressure on the administration to lobby more forcefully for an end to the brutal war, which has left more than 10,000 dead more than 3 million displaced. Many of those, including infants, their mothers and school-aged children, are now malnourished, in desperate need of life-saving aid and pleading for a halt to the conflict.

“I’m hopeful that this is a leverage moment that the administration uses to get a cease-fire,” Murphy continued.

Amend and kill

With the next procedural vote coming later this week, opponents are now exploring various ways to kill the measure. Their main strategy is to open the legislation up to amendments. Any controversial amendment that passes this week risks peeling off a vote from the left or the right on the legislation’s final passage. That’s why supporters are calling for a limited number of amendment votes, though they’re ultimately hoping none will be allowed — even their own.

“The easiest way to deal with this, it seems to me the only sensible way, is to have a clean vote on the Lee-Sanders. Just we all agree no amendments – just let it go up or down. To me that’s the simplest and the fairest way to handle it,” Sen. Ben Cardin (D-Md.) told reporters at the Capitol. “I have a lot of foreign policy issues I would like to get passed in this Congress. This is not the vehicle to do them in.”

“We have to do something about what happened to Khashoggi”

Still, many powerful Republicans are working to scrap the resolution that would pull the U.S. military completely out of the conflict. They want to start over and, as they argue, use more precise, targeted means to punish the Saudi regime.

“We have to do something about what happened to Khashoggi and about recalibrating our relationship with the Saudis, but the underlying resolution is the wrong way to do the right thing,” Sen. Marco Rubio (R-Fla.) told reporters after the vote.

Republican opposition

Rubio argues American officials have other options at their disposal, including sanctioning top Saudi officials, restricting the investments U.S. companies can make in the nation and even going around Saudi leaders and reaching out to Qatar as an ally on strategic issues in the region.

Other top Trump allies argue the current proposal is too blunt of an object as it stands, so they’re lobbying their fellow Republicans — those who up until now have merely voted to debate the bill on the Senate floor — to now oppose it.

“I want to see us express the strongest disapproval for the Khashoggi killing, but I also want us not to surrender Yemen to Iran in basically a proxy war,” Sen. John Cornyn (R-Texas) told VICE News while waiting for the tram under the Capitol.

Other war hawks, like Trump ally Sen. Graham, say they can be put at ease if the administration relents and sends CIA Director Haspel to brief senators before the next round of voting starts.

Others say that’s not good enough.

“I’m not so hung up on the CIA presentation having to be made. I’m more hung up on how we’re addressing an out-of-control crown prince,” outgoing Sen. Bob Corker (R-Tenn.), who chairs the Foreign Relations Committee, told VICE News on his way to a vote in the Capitol. “I don’t want us to cut our nose off to spite our face, but we haven’t achieved the balance between American interests and American values yet, and that’s what I’d like us to do with this legislation.”

The death of former President George H.W. Bush — who led the CIA before being sent to the Oval Office — is also delaying this week’s debate. But for now, the legislation’s main sponsors feel more empowered and emboldened than ever in the three years that they’ve been crying out to get the U.S. out of the humanitarian quagmire that’s devastated Yemen.