As the possibility of armed conflict with Iran grows more real, lawmakers in Congress are struggling to settle on what—if anything—they are obligated to do as the only branch with the constitutional authority to declare war.

Many lawmakers, including a odd-couple coalition of libertarian-minded Republicans and mainstream and progressive Democrats, are increasingly worried that the Trump administration might use, as a legal basis for war, the 18-year old authorization of military force, or AUMF, that Congress approved immediately after the September 11th attacks. And as the possibility of conflict inches closer, they are making a play to force the administration to come to Congress and actually convince them military action is necessary.

In the Senate, Sen. Tim Kaine (D-VA) and several others, including Sen. Rand Paul (R-KY) filed an amendment last week to the annual Department of Defense spending authorization that would block funds for a conflict with Iran unless Congress explicitly authorizes military action.

“The administration desperately wants to avoid coming to Congress on this, and it looks like they’re constructing an argument, the purpose of which is to avoid Congress,” said Kaine. “The purpose is not really to make a great argument about the 2001 AUMF.”

And in the House, Rep. Ro Khanna (D-CA) and Rep. Matt Gaetz (R-FL) announced on Monday that they are introducing a resolution that would block the administration from going to war with Iran on the basis of the 2001 AUMF. Such a measure would require Trump to obtain explicit congressional approval for any hostilities with Iran. Khanna told The Daily Beast that their resolution will likely make it to the House floor next week as an amendment to the House’s Pentagon authorization bill.

“You've got a number of people on both sides who are concerned about this endless war,” Khanna said. “I don't think even this administration will stretch to make a case that the 2001 AUMF covers Iran, and if they did, that's why this amendment is so important, to clarify that it doesn't.”

The 11th hour legislative moves surrounding the AUMF mark the latest test of Congress’ will to stand up to the Trump administration on matters of war and foreign policy. With escalatory acts unfolding in the region—this week, U.S. officials blamed attacks on oil tankers in the Persian Gulf on Iran—a sense of urgency has imbued lawmakers’ efforts to reassert their constitutional role. That urgency has been compounded by hints from the administration that it believes it has the authority under the 2001 law to conduct military exercises—and not just defensive measures—in Iran.

In the past, Secretary of State Mike Pompeo has linked Iran with al Qaeda—which is specifically named in the 2001 AUMF—and claimed that the Iranian government has supported them. That claim is hotly contested by Iran experts.

Kaine, a Senate Armed Services Committee member and vocal critic of the 2001 AUMF’s continued use, said the administration has made clear enough they think the 2001 authorization “would enable them to wage war against Iran without coming to us.”

It was clear during a hearing in the House Armed Services Committee last week that Pompeo, during a closed-door May briefing , left some lawmakers with the impression that he believed the 2001 AUMF was a sound legal justification for actions in Iran. Though Rep. Mike Turner (R-OH) asserted during the hearing that “no one” in the administration suggested that using the 2001 AUMF for Iran was on the table, Gaetz said that such a claim was “not consistent with my understanding of what they said to us.” And Rep. Elissa Slotkin (D-MI) said that “we were absolutely presented with a full formal presentation on how the 2001 AUMF might authorize war on Iran.”

Slotkin told The Daily Beast on Tuesday that Pompeo’s remarks did not include a direct reference to the AUMF. “What he did was, he raised the relationship between al Qaeda and Iran,” she said. To her, that, combined with Pompeo declining to rule out using the 2001 AUMF in testimony before the Senate Foreign Relations Committee in April, was tantamount to a case from the administration.

In response to questions, a spokesperson for Pompeo pointed The Daily Beast toward the secretary’s appearance on CBS’ Face the Nation on Sunday, during which he said “the American people should be very confident the actions that the United States takes under President Trump will always be lawful, always consistent with our Constitution, and we will always do the hard tasks it takes to protect American interests wherever they are.” Pompeo didn’t answer a follow-up from Margaret Brennan about whether congressional permission would be required.

Were the administration to cite the 2001 AUMF for legal cover for military operations in Iran, the likelihood is that many members of Congress would not object. Past presidents have used the same law to launch military ventures in Afghanistan, Iraq, Libya, Syria, Somalia, Yemen, and elsewhere since September 2001. And most lawmakers have given them leaway to do so.

The coalition of lawmakers who have insisted that the 2001 AUMF is being mangled and misused well past the point of recognition has grown in recent years. Rep. Mac Thornberry, the top Republican on the House Armed Services Committee, said during last week’s hearing that he did not believe, “for what it’s worth, that the 2001 AUMF authorizes military force against the state of Iran.” And Gaetz, a close ally of the president, has injected a Trumpian line of argument into the discussion, asserting that war with Iran clashes with the spirit of Trump’s “America First” foreign policy and is arguing funds are far better spent at the U.S.-Mexico border.

But the majority of lawmakers have not expressed public opposition to using the 2001 AUMF and others have sidestepped the question altogether by claiming that U.S. military action would be justified if it were in response to an attack.

“I don’t think an AUMF would be necessary for self-defense,” said Sen. John Cornyn (R-TX). “I’m sure Congress won’t be silent… I think we certainly have the authority to get involved.”

The next indication of the type of pushback the administration may face from the Hill could come on Wednesday, when Brian Hook, the State Department’s lead on Iran policy, appears before the House Foreign Affairs Committee. A Democratic lawmaker told The Daily Beast that they expect Hook to get grilled on the AUMF issue.

Already, there’s plenty of appetite for clarification on the administration’s Iran stance among lawmakers.

“If you’re not intending to try to use the 2001 AUMF to go to war with Iran, the easiest way to clear that up would be to say, we recognize we cannot use the 2001 AUMF aimed at the perpetrators of 9/11 to go after Iran,” Rep. Abigail Spanberger (D-VA) told The Daily Beast.

But Spanberger, a former CIA terrorism specialist, said no such clarification has come. “No one has explicitly answered that question,” she said. “In fact, that question has been wholeheartedly avoided.”

—with reporting by Erin Banco