While Sanders prioritized domestic issues over foreign policy concerns during his 2016 presidential bid, today’s bill may foreshadow a greater willingness by 2020 frontrunners to question open-ended military support for Middle Eastern allies. Sen. Chris Murphy, D-Conn., a vocal Saudi critic and potential 2020 contender, notably joined Sanders and Lee in introducing the resolution.

“For far too long, Congress under the Democratic and Republican administrations has abdicated its constitutional role in authorizing war,” Sanders said. “The time is long overdue for Congress to reassert its constitutional authority.”

“Many Americans are unaware that the people of Yemen are suffering today in a devastating civil war with Saudi Arabia and our allies on one side and Houthi rebels on the other,” Sanders said at a press conference introducing the resolution. “Many Americans are also not aware that US forces have been actively involved in support of the Saudis in this war providing intelligence and aerial refueling of planes whose bombs have killed thousands of people and made this crisis worse.”

Sanders, an independent, rose to national prominence as a left-wing firebrand for his willingness to challenge Democratic Party orthodoxy on domestic issues during the 2016 Democratic primaries. Anti-war activists hope that the backing of the high-profile progressive and potential 2020 candidate for president will lend new momentum to their efforts to end US support for an indiscriminate bombing campaign that has been blamed for the deaths of hundreds of civilians.

Progressive Sen. Bernie Sanders of Vermont joined forces today with Tea Party conservative Sen. Mike Lee, R-Utah, on the latest bipartisan resolution to end US support for the Saudi-led war against the Houthis in Yemen.

“This will be a telling moment for the Democratic caucus to see where they are on some of these questions,” Stephen Miles, the director of the activist coalition Win Without War, told Al-Monitor. “It should not divide the [Democratic] caucus whether or not the US continues supporting a war that’s causing the largest humanitarian crisis on the planet.”

A January poll from the Committee for Responsible Foreign Policy, which advocates for increased congressional oversight of US military interventions, found that 51.3% of voters would be less likely to vote for their congressional representatives “if they did not act to withdraw from conflict in regions abroad, such as Yemen.” The poll also found that 67.4% of voters “disapprove of congressional leadership allowing our involvement in conflict overseas without formally approving military action.”

The Sanders-Lee resolution is just the latest congressional pushback against Riyadh’s Yemen campaign. Rep. Ro Khanna, D-Calif., introduced similar legislation last year with the backing of the Congressional Progressive Caucus, which helped organize a background briefing on the Senate bill with Sanders’ and Lee’s offices Tuesday.

Both versions call on Trump to remove US armed forces in Yemen except for those engaged in operations against al-Qaeda and the Islamic State. US troops aren’t engaged in combat against the Houthis but the United States has been providing refueling support for Saudi and Emirati bombers since the Barack Obama administration. The resolution’s backers argue that US refueling qualifies as hostilities since it places US forces in harm’s way.

The Department of Defense, however, is already pushing back. The Huffington Post first reported that the Pentagon sent a letter to every Senate office today opposing the resolution and arguing that US support for the Saudis “does not constitute ‘hostilities.'”

“The limited military and intelligence support that the United States is providing the [Saudi]-led coalition does not involve any introduction of US forces into hostilities,” the letter said.

But Sanders and Lee are confident that the US refueling services and targeting assistance for the Saudi coalition qualify as hostilities under the War Powers Act, which would give their resolution expedited consideration for a Senate floor vote.

“The War Powers Act does trigger our authority whenever we put our armed services personnel into a situation where imminent involvement in hostilities is clearly indicated by the circumstances,” Lee told Al-Monitor. “We’re also putting our armed forces personnel and equipment into territory, airspace or waters of a foreign nation while preparing for combat. Those things themselves implicate the War Powers Act.”

When Khanna made a similar argument in the House, House leadership successfully fought to keep Khanna’s resolution from coming to the floor until he agreed to make it nonbinding and emphasize Iran’s support for the Houthis.

It remains unclear whether Senate leadership could stave off a floor vote. Either way, the cooperation between lawmakers on the two ends of the political spectrum is giving anti-war activists renewed hope.

“The two of them working together is certainly going to attract a lot of media attention and there’s been a lot of grassroots support for this issue,” said Kate Gould, the legislative director for Middle East Policy at the Friends Committee on National Legislation, a Quaker lobby.

While both resolutions are technically bipartisan, 47 of the 50 co-sponsors in the House are left-leaning Democrats. And Lee, the Republican co-sponsor of the Senate resolution, is a frequent critic of President Donald Trump. Yemen war critics, however, are quick to point out a surge in anti-war activism from Tea Party and Libertarian-leaning groups as well.

“We know there’s a lot of grassroots support from both the right and the left and everywhere in between,” Gould told Al-Monitor. “We saw with Mr. Khanna’s resolution, there was support from … Tea Party-affiliated groups.”

Lawmakers have also increasingly sought to staunch the flow of US weapons to the Saudis as the civilian toll and damage to Yemen’s infrastructure mount.

Last year, Murphy and Sen. Rand Paul, R-Ky., pushed for a vote to block a $500 million sale of precision-guided missiles for use against the Houthis. Senate Minority Leader Chuck Schumer, D-N.Y., supported blocking the sale. The vote failed by a vote of 53-47 — far closer than a similar effort by the two senators to block a $1.15 billion tank sale in 2016.