Anti-war groups on Friday are urging their supporters to pick up the phone to tell U.S. senators to back a joint resolution to "end America's shameful role in Yemen."

The Sanders-led resolution, introduced at the end of last month, calls for "the removal of United States Armed Forces from hostilities in the Republic of Yemen that have not been authorized by Congress."

The United States has been fueling the conflict for years by aiding Saudi Arabia's bombing campaign with weaponry and military intelligence, leading to accusations by rights groups and some lawmakers that the U.S. is complicit in fueling what the United Nations describes as "the world’s largest humanitarian crisis."

There is urgency for constituents to make the calls, the groups warn, as a vote could come as soon as Monday.

In a further push to make the resolution successful, Win Without War led a group of over 50 organizationse—including CODEPINK, Democracy for America, Our Revolution, and War Resisters League—in sending a letter Thursday to senators calling on them to back the resolution.

Their letter says that "U.S. weapons sold to Saudi Arabia have been misused repeatedly in airstrikes on civilians and civilian objects, which are the leading cause of civilian casualties in the conflict and have destroyed Yemen's vital infrastructure. This destruction of infrastructure has exacerbated the world's largest hunger crisis in which 8.4 million civilians are on the brink of starvation and created the conditions necessary for the largest cholera outbreak ever documented in modern history," they state.

"Congress has a constitutional and ethical duty to ensure any and all U.S. military operations comply with domestic and international law, and U.S. participation in the civil war in Yemen raises numerous legal and moral questions that must be resolved by Congress," the letter continues.

"With S.J.Res. 54, the Senate must send a clear signal that without congressional authorization, U.S. military involvement in Yemen’s civil war violates the Constitution and the War Powers Resolution of 1973," it adds.

It wasn't the only letter senators received Thursday calling on them to support the resolution.

A group of nearly three dozen experts—including former U.S. ambassador to Yemen Stephen Seche and Nobel peace laureate Jody Williams—also delivered a similar missive to lawmakers.

In their letter, the group of experts referenced an assessment by Reps. Ro Khanna (D-Calif.), Mark Pocan (D-Wis.), and Walter Jones (R-N.C.), which said, in part:

Nowhere else on earth today is there a catastrophe that is so profound and affects so many lives, yet could be so easy to resolve: halt the bombing, end the blockade, and let food and medicine into Yemen so that millions may live. We believe that the American people, if presented with the facts of this conflict, will oppose the use of their tax dollars to bomb and starve civilians.

The resolution currently has 8 co-sponsors, including one Republican, Mike Lee of Utah. The Democratic senators co-sponsoring the resolution are Chris Murphy of Connecticut, Cory Booker of New Jersey, Dick Durbin of Illinois, Elizabeth Warren of Massachusetts, Ed Markey of Massachusetts, Patrick Leahy of Vermont, and Dianne Feinstein of California.