WASHINGTON — To override President Donald Trump’s veto of a measure to end U.S. military support for Saudi Arabia’s war in Yemen, Sen. Bernie Sanders would have to get 13 more Republicans to change their votes for the supermajority needed.

Now Sanders, one of many Democratic presidential candidates, is betting that more of his fellow senators will stand up to defend Congress as an institution and its constitutional authority to declare war. On Monday, he issued a broad call that they support a veto override vote.

”The president’s action is a very serious challenge to congressional authority that demands a response," Sanders, I-Vt., wrote in a letter to his fellow senators. “For far too long Congress, under both Democratic and Republican administrations, has abdicated its constitutional role with regard to the authorization of war … Congress must now act to protect that constitutional responsibility by overriding the president’s veto.”

The Sanders camp believes the joint resolution’s privileged status will guarantee a Senate floor vote on the veto override, but he would need 67 votes for the override to succeed.

Last time, on March 13, six Senate Republicans joined all the Democrats to pass the joint resolution, 54-46. The House passed it 247-175 on April 4.

Should US troops be involved in the Yemen civil war? House lawmakers voted to block U.S. military from helping Saudi allies from conducting airstrikes and other missions in the war-torn country.

Though Trump, with the second veto of his presidency, rejected the bipartisan joint resolution, its passage was considered a milestone for lawmakers, who invoked the War Powers Resolution for the first time since its passage in 1973.

With the veto, the White House argued not only that the Yemen measure’s passage would weaken efforts to fight al-Qaida and Islamic State affiliates but also that Congress was overstepping its authority and impinging on the president’s constitutional powers as commander in chief.

× Fear of missing out? Sign up for the Early Bird Brief, the defense industry's most comprehensive news and information, straight to your inbox. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

A Sanders aide acknowledged that flipping more than a dozen Republicans would be a heavy lift, but the senator’s hope is that lawmakers will want to push back on the White House’s argument and continue to “shake the cobwebs off” in asserting Congress’s constitutional war powers.

Since 2015, the U.S. has provided limited support to member countries of the Emirati and Saudi-led coalition fighting against Iran-backed rebels in Yemen, including intelligence sharing, logistics and, until late last year, aerial refueling. The U.S. also provides billions of dollars in U.S.-made weapons to the kingdom.

Congress has grown uncomfortable with the humanitarian crisis in Yemen as well as Trump’s close relationship with Saudi Arabia as he tries to further isolate Iran, a regional rival — especially after the killing of Jamal Khashoggi, a Saudi who lived in the U.S. and had written critically about the kingdom.

Pentagon watchdog to investigate Yemen refueling The Pentagon’s internal watchdog will investigate military refueling missions in the Mideast and Africa after the U.S. undercharged allies by $331 million for its support in the Yemen civil war.

Win Without War director Stephen Miles, whose organization backs the resolution, speculated the veto-override vote may break slightly differently than the previous vote. That’s because it’s now about whether Congress, which never authorized the war, should be allowed to be ignored by the executive branch, he said.

“I’d argue, as some others have, that the bar for Congress exerting its constitutional role cannot be so high as to allow the executive [branch] to block it without a supermajority,” Miles said.

It’s unlikely that enough Senate Republicans would make the switch for a successful override, said Miles, blasting them for placing loyalty to the president ahead of the institution of Congress.

If it fails, Sanders and other supporters could try to attach similar legislation to must-pass defense spending or policy bills, which would test the president’s resolve when they appear for his signature. Or supporters could sue.