Collins, who does not sit on the committee, was one of only five Republicans to vote in favor of cutting US support to the Saudi coalition last month out of concern for the growing number of civilian casualties in what has become the world’s largest humanitarian crisis. The new bill , spearheaded by Sens. Todd Young, R-Ind., and Jeanne Shaheen, D-N.H., looks set to gain traction with more Republican support.

“I believe that Congress needs to play a more aggressive role in overseeing our military in that mess,” Sen. Susan Collins, R-Maine, one of the four original co-sponsors of the bill, told Al-Monitor.

Last month, the Senate voted 55-44 to kill legislation to end US support for the war against the Iranian-backed Houthis. Now the Foreign Relations Committee is on the verge of advancing a new bill that would force Riyadh to meet certain criteria in order to qualify for US refueling support that is essential to carrying out Saudi Arabia's bombing campaign.

While many lawmakers of both parties agree that the commander in chief has broad authority to engage in such short-term actions, they are under growing pressure to play a more forceful oversight role. One new bill under discussion would supplant a counterterrorism law from 2001 that has been used to justify an ever-expanding number of interventions in the Middle East and beyond, while another could complicate US support for the Saudi-led campaign in Yemen.

“From my perspective, [the bill] is getting to a very, very good place,” committee Chairman Bob Corker, R-Tenn., said at a hearing on Yemen earlier this week. Corker, who voted against ending US support for the Saudis last month, promised a committee vote on the bill by the end of May.

Unlike Shaheen, Young voted against the blanket cutoff in military assistance for the Saudi campaign last month even though he has been a vocal critic of Saudi Arabia’s chokehold on the entry of humanitarian aid and commercial goods into Yemen. Anti-war activists initially criticized both senators for offering a bill they viewed as ineffectual and detracting from last month’s floor vote, but they now credit that vote for recent changes that strengthen the bill.

“The Senate is circling in on something that will restrict US support for the Saudi war and that’s been the message all along,” Stephen Miles, the director of Win Without War, a coalition of anti-war groups, told Al-Monitor. “US support for this war is not unlimited and there are real deep concerns. If Saudi Arabia does not address those concerns, they will lose US support.”

Young’s revised bill would end US midair refueling support for the Saudi coalition unless the secretary of state certifies that Riyadh meets a series of criteria. It also removes a broad exemption that would have allowed the Trump administration to continue military support absent a certification in order to combat “Iranian terrorist activities in Yemen.”

For US support to continue, the bill now requires the Saudis to engage in “an urgent and good faith effort to conduct diplomatic negotiations to end the war in Yemen,” allow “food, fuel and medicine” through key border crossings and to reduce civilian casualties by complying with “applicable agreements and laws regulating defense articles purchased or transferred from the United States.”

Last year, Sen. Rand Paul, R-Ky., attempted to block a $500 million sale of precision guided missiles to Riyadh, but the measure was narrowly defeated on the Senate floor. Paul told Al-Monitor that he’s “looking at” a similar measure attempting to block another $2 billion worth of Saudi arms sales announced by the State Department in recent weeks. The pending sales include $1.3 billion in self-propelled howitzers, $670 million in anti-tank missiles and $300 million in spare parts to repair Abrams tanks and other military vehicles.

Paul also joined a handful of other lawmakers who have questioned the legality of Trump’s strikes on the Assad regime absent authorization from Congress.

“By illegally bombing Syria, President Trump has once again denied the American people any oversight or accountability in this endless war,” Rep. Barbara Lee, D-Calif., tweeted after the strikes. “Congress, not the president, has the power to authorize military action.”

Paul and Lee have long tried to push for a repeal of the 2001 Authorization for the Use of Military Force used against al-Qaeda and the Islamic State (IS) in order to prompt a debate on a new war authorization bill. Corker and Sen. Tim Kaine, D-Va., this week released a new bill that they soon hope to advance as a replacement to the 17-year-old law, but it has faced pushback from some Senate liberals and anti-war activists who view the bill as a further rollback of Congress’ ability to authorize war.

The Corker-Kaine proposal would allow the administration to continue counterterrorism operations against al-Qaeda, IS, the Taliban and “associated forces” in Afghanistan, Iraq, Syria, Somalia, Yemen and Libya. While Corker had hoped to mark up the bill next week, Sen. Bob Menendez, D-N.J., the top Democrat on the Foreign Relations Committee, has asked for more public hearings from administration officials and outside experts to review the proposal.

Activists such as Miles have expressed significant concerns about the Corker bill, arguing that the only way Congress could stop the administration from expanding the definition of “associated forces” or the scope of countries where operations can take place is with a veto-proof majority.

“It’s the main problem with letting the administration say which groups they’re fighting,” said Miles. “If Congress wants to force the issue that can take a month, which, in the world of modern warfare, is a lifetime.”