Sen. Bernie Sanders on Monday called on his fellow members of Congress to come together and override President Donald Trump’s veto of the historic Yemen War Powers resolution.

“The president’s action is a very serious challenge to congressional authority that demands a response,” the Vermont senator and 2020 presidential contender said in a dear colleague letter.

The Yemen resolution aims to end U.S. complicity in the world’s worst humanitarian catastrophe. Its passage of the House and Senate marked the first time Congress has invoked the 1973 War Powers Act.

“For far too long Congress, under both Democratic and Republican administrations, has abdicated its constitutional role with regard to the authorization of war,” added Sanders, the lead Senate sponsor of the Yemen measure. “The Congress must now act to protect that constitutional responsibility by overriding the president’s veto.”

Trump’s veto of the bipartisan Yemen resolution last week, as Common Dreams reported, was widely denounced by anti-war groups and progressive lawmakers.

In a statement following the president’s veto, Congressional Progressive Caucus co-chairs Pramila Jayapal (D-Wash.) and Mark Pocan (D-Wis.) said Trump “signed the death warrant of countless innocent Yemeni men, women, and children.”

“Congress refuses to accept the president’s decision to condemn Yemeni civilians to unspeakable suffering and carnage,” Jayapal and Pocan said. “Despite this setback, the Progressive Caucus will pursue every legislative means of ending this immoral conflict, including a possible vote to override the president’s veto.”

The Senate passed the Yemen resolution last month by a 54-46 vote. With every senator in attendance, the resolution would need 67 yes votes to override the president’s veto.

In the House—where the resolution passed 247-175 earlier this month—the resolution would need 290 votes.

Sanders urged senators who support U.S. military intervention in Yemen to “bring that perspective to the floor of the Senate, debate the issue, and call for a vote.”

According to the humanitarian group Save the Children, at least 85,000 young Yemeni children have died of starvation over the past three years as a result of U.S.-backed Saudi bombing. The United Nations said last October that half of the population of Yemen is on the brink of famine.

“At the end of the day, however,” concluded the Vermont senator, “let us agree that it is imperative that Congress reaffirm the power given to us by the Constitution over matters of war, one of the most serious duties we have as members of Congress.”

Read Sanders’s full letter: