A group of constitutional scholars and lawmakers want House Speaker Nancy Pelosi to take President Donald to the Supreme Court over the war in Yemen.

Their case is straightforward: Trump is unilaterally involving the United States in war, and that’s unconstitutional. For four years, the United States has participated in a war in Yemen that was never authorized by Congress and that Congress expressly told Trump to withdraw from. Trump ignored the directive. Now, as the White House escalates tensions with Iran, there’s growing concern that unless legal action is taken, Congress will cede more war powers to Trump.

In April, Congress passed a historic War Powers Resolution directing Trump to remove troops involved in “hostilities” in Yemen. Trump vetoed it, cementing American fingerprints on one of the worst humanitarian crises in the world: According to the most recent United Nations report, 80 percent of the Yemeni population — 24 million people — is in need of humanitarian assistance. The Senate failed to reach the 67-vote threshold needed to override the executive veto on the bill.

Trump said the War Powers Resolution was an attempt to “weaken [his] constitutional authorities.” But the power to authorize a declaration of war, of course, sits with Congress, not the president.

Constitutional scholars are now arguing a War Powers Resolution isn’t a normal bill. They say it is a fundamental constitutional question about the power to authorize war. And the stakes are high.

“The president’s veto doesn’t end this conversation,” Bruce Ackerman, a constitutional law scholar with Yale University, told Vox. He’s one of a diverse group of legal experts who have sent Pelosi a letter urging her to take legal action.

The United States got involved in Yemen four years ago when Saudi Arabia and its allies began a military campaign in Yemen against Iranian-backed Houthi rebels. The US is providing Saudis with intelligence, arms and ammunition, and, until late last year, fuel for their warplanes. The warplanes that bombed a school bus, killing at least 40 children last August, did so with an American-made bomb.

The war has killed more than 50,000 people, according to one independent estimate, and has left tens of millions more in need of assistance. Trump is determined to keep course, and has shown he is committed to his alliance with Saudi Crown Prince Mohammed bin Salman, even after it became clear the prince called for Saudi Washington Post journalist Jamal Khashoggi’s killing.

Since Trump’s veto, his administration has escalated tensions with Iran, spreading concern that Trump is itching to stoke another war. These concerns were exacerbated on Friday, when the administration sidestepped Congress again, unilaterally authorizing $8 billion in arms sales, including to Saudi Arabia and its allies, to counter Iran.

“This is a moment of truth, both for the congressional war power and for the Supreme Court of the United States,” Ackerman said. “Does the Supreme Court of the United States — and its claim of originalism — is that supposed to be taken seriously?”

The case for taking Trump to the Supreme Court, briefly explained

Constitutionally, Congress has the power to declare war, and the president, as the commander in chief, has the power to direct the military after Congress’s authorization. Historically, America’s war-making has gone differently; presidents have consistently pushed the boundaries of their power.

For example, for the past 18 years, presidents of both parties have used the same 2001 congressional war authorization, passed after 9/11, as justification for going to war across the Middle East, including Yemen. The Obama administration did not ask Congress for specific authorization before involving itself in Yemen. The Trump administration hasn’t either, bypassing the legislative branch as it negotiated billions of dollars in arms deals, and as it conducted military operations.

Instead of seeking congressional approval for war, presidents have increasingly turned to their internal legal counsels to build a largely secret body of law to defend their war engagements, Mary Dudziak, a constitutional scholar with Emory University School of Law, said.

“Especially since 9/11, Congress has let power accumulate in the executive branch,” Dudziak said. “And Congress does that by sitting back and not objecting, which is why Pelosi’s role is really important right now.”

Now, Dudziak and others say Congress has an opportunity to fight back. Legal scholars are pointing to a 1950s Supreme Court case, Youngstown Sheet & Tube Co. v. Sawyer, to make the case to Democratic leadership that Congress has the winning argument against Trump on war powers.

In 1952, during the Korean War, a labor strike stalled steel production, cutting the availability of war materials for American troops. So President Harry Truman invoked his emergency powers to seize private steel mills to force steelworkers back, blatantly defying specific provisions in the Taft-Hartley Act — a largely anti-union labor law from 1947. Truman’s justification was that he could not wage a successful war without the proper materials, and therefore was allowed to act outside the law.

The Supreme Court rejected Truman’s case, and, importantly, Justice Robert Jackson’s opinion established a clear three-tier framework for presidential powers. He wrote that a president’s powers are at maximum strength only with Congress’s authorization. When Congress shows indifference, presidential power is in a “twilight zone,” as both branches can claim authority over a given matter. Finally, a president’s power is “at its lowest ebb” when Congress has specifically disapproved of an action.

US involvement in the war in Yemen firmly sits in this third category. Congress has passed a War Powers Resolution — a form of legislative action established in 1973 to prevent another Vietnam War — specifically directing Trump to withdraw from Yemen. In passing the resolution, Congress not only tried to claw back its constitutional powers from the president but also asserted that the executive had overstepped his bounds as commander in chief, going to war without Congress’s approval.

“We have the leading fundamental precedent on this requiring the Supreme Court to scrutinize presidential war-making,” Ackerman said. “If Nancy Pelosi doesn’t go to court and urge the Supreme Court to intervene when Congress has for the first time invoked this [War Power Resolution], in the case where neither President Obama nor President Trump has gained the approval, it establishes a precedent that this war power shouldn’t be taken seriously.”

Do Democrats want to get into this fight?

Since Trump’s veto, lawmakers and activists have been waiting for their next move on Yemen.

“We continue to consider all viable options to end this humanitarian crisis,” Pelosi’s spokesperson said.

Rep. Ro Khanna (D-CA), who shuttled the War Power Resolution through the House, with Sen. Bernie Sanders (I-VT) doing the same in the Senate, is hopeful: “It wasn’t a no,” he told Vox of leadership’s interest in taking Trump to court. That said, there still isn’t consensus that this is the best strategy going forward.

Sanders told Vox he would be supportive of Pelosi taking legal action, but said his primary focus is on building enough support that the Senate can overturn Trump’s veto. Sanders often talks about Yemen, and the importance of Congress’s war powers (especially with respect to Trump’s escalations with Iran), on the national stage as one of the leading 2020 Democratic presidential candidates.

“I think essentially we have got to rally the American people to understand what a horrible humanitarian disaster this is, and we need the votes to override Trump’s veto — that is to me the major issue,” Sanders said. “The court case would go on and on. And we need immediate action.”

Dudziak acknowledged the need for both legal action and a national campaign to gain Americans’ attention.

“The only way for Congress to have any role in restraining president’s war power is for Congress to act,” she said. “The hardest thing to [doing] anything is profound apathy about our conflict on the part of the American people.”