President Donald Trump’s veto of a joint resolution to put an end to U.S. support for the Saudi-led war in Yemen is going to kick the matter back to senators when they return to the Capitol next week.

With recess continuing this week, senators are in their home states, on congressional delegations abroad and on the presidential campaign trail. But Vermont independent Sen. Bernie Sanders , one of the many Democratic presidential candidates, took a moment Monday to circulate a dear colleague letter seeking support for overriding the Trump veto.

“For far too long Congress, under both Democratic and Republican administrations, has abdicated its Constitutional role with regard to the authorization of war,” Sanders wrote to his Senate colleagues. “The historic passage of this resolution, the first time since the 1973 War Powers Resolution was passed that it has been successfully used to withdraw the United States from an unauthorized war, was a long overdue step by Congress to reassert that authority.”

“The Congress must now act to protect that constitutional responsibility by overriding the president’s veto,” Sanders wrote.

Sanders, who has been out on the presidential campaign trail himself, wrote in the letter that supporters of the American involvement in Saudi Arabia’s Yemen campaign should seek a floor debate and and vote on that effort.

The resolution gained significant momentum last year in the aftermath of the murder of journalist Jamal Khashoggi inside the Saudi consulate in Istanbul, Turkey.

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The Senate is likely going to have to take up the veto override vote, though given that the underlying resolution only passed with a 54-46 vote under an expedited process, it its not expected to prevail.

There are a number of procedural options available to senators for dispensing with veto messages. Motions to refer the president’s message to committee or to table the matter entirely are possible (though unlikely to work given the support of a majority of senators for the resolution itself).

While it’s unusual to try to force a cloture vote on a veto override effort, it would be possible to debate the Yemen resolution to death since the 60 votes needed for cloture is a greater number than the number of senators supporting the joint resolution.

The most straightforward way to dispense with the Yemen resolution would be to reach a unanimous consent agreement to set up a direct vote on overriding the Trump veto, which would require 67 votes if all of the senators are in attendance that day.

The president’s message, which was released oh April 16, contends that the War Powers resolution is not necessary since U.S. troops are not directly engaged in Yemen except for possible counter-terrorism operations. Trump’s message also said that the proposed restrictions would would be dangerous and counterproductive to foreign policy.

“The Congress should not seek to prohibit certain tactical operations, such as in-flight refueling, or require military engagements to adhere to arbitrary timelines,” the message said. “Doing so would interfere with the President’s constitutional authority as Commander in Chief of the Armed Forces, and could endanger our service members by impairing their ability to efficiently and effectively conduct military engagements and to withdraw in an orderly manner at the appropriate time.”

Regardless, the Democratic-led House of Representatives in not expected to ever get a chance to see the veto message, since it’s on a joint resolution that was first introduced in the Senate.