No one was more surprised than Rep. Barbara Lee (D-Calif.) on Thursday when her language revoking the administration's war authority was unexpectedly backed by Republicans and added to a must-pass defense spending bill.

“Whoa,” Lee wrote on Twitter following a voice vote that pushed through her amendment to sunset the 2001 Authorization for Use of Military Force (AUMF).

Lee’s measure, which prompted applause when it was adopted in the House Appropriations defense bill, would revoke the AUMF eight months after the passing of the defense act, forcing Congress to vote on a new law in the interim.

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Lee for years has attempted to shutter the law passed in the wake of the Sept. 11 terrorist attacks, but this week’s backing reflects shifting politics and unusual bipartisan support.

GOP lawmakers, growing more frustrated with years of unresolved military conflict, are now pushing to create a new war bill specific to current conflicts.

“I feel like my world is rocked, because I see these very different opinions and yet I agree with you,” Air Force veteran Rep. Chris Stewart Christopher (Chris) Douglas StewartAtlanta Wendy's 911 call the night of Rayshard Brooks's death released Tyler Perry offers to pay for funeral of Rayshard Brooks Current, former NHL players form diversity coalition to fight intolerance in hockey MORE (R-Utah) told Lee during the amendment’s debate.

“My friends in the military now ... they notice that Congress doesn’t have the guts to stand up and have this debate and give them the authority with their continuing every day,” he added.

Stewart later told The Hill he has tried in the past to convince fellow Republicans to discuss a new AUMF, and the new shift is likely the result of having a new president in the White House.

“President Obama wasn’t interested in expanding this authority and he wasn’t interested in this debate,” he said. “Many of us believe we have a president that is more likely to help us on this rather than resist.”

The AUMF has been used by the George W. Bush, Obama and Trump administrations to justify a number of military actions, including the Iraq War and the fight against the Islamic State in Iraq and Syria.

While Libertarian outliers including Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) have pushed for a new war authority — arguing that any president needs Congressional authorization for military action — lawmakers have been stalemated for years amid myriad political and policy divisions.

Rep. Tom Cole (R-Okla.), the chairman of the House Appropriations Subcommittee on Labor, Health and Human Services, argued that the time is now for Congress to debate a new measure.

“We’re at war against an enemy that did not exist in a place that we did not expect to fight. How an AUMF that was passed 16 years ago — before I was in Congress — could possibly be stretched to cover this is just beyond belief to me," Cole said Thursday during debate.

Conservative experts weren’t able to point to one specific example that tipped Republicans in favor of an AUMF debate this time around, and said the outcome was likely the result of a variety of frustrations.

Judson Phillips, the founder of conservative advocacy coalition Tea Party Nation, said “one of the sentiments among the grassroots has been that Congress has abdicated too much of its power to the executive branch.”

“While the Afghan authorization issue is not something that's specifically on a lot of people’s radar, government overreach and the power of the presidency is,” he said.

“This is an issue that would pick up some traction among the grassroots,” he added.

A.J. Spiker, the former chairman of the Iowa Republican Party who served as a senior political adviser to Paul’s leadership PAC, said the 2001 war authorization was passed when many of today’s troops were still in elementary school.

“A lot of the soldiers that are now going off to fight in these wars they were not even in middle school at the time of 9/11. Here they are being sent off with a war authorization from two presidents ago,” he said.

“Nobody thought an authorization for war would last 15 years. That’s just ridiculous. I think that’s something Democrats and Republicans should be able to come together on,” Spiker said

Recent events in Syria — including the U.S. downing a Syrian drone and Russian aircraft within the same week earlier this month — have also caused concern over the authority, according to retired Army Lt. Gen. Thomas Spoehr of the conservative Heritage Foundation.

“The U.S. military on the ground in Syria is more than just special operations — that’s raised some questions in Congress’ mind,” Spoehr told The Hill. “All these things coming together tilted the board.”

Regardless of the cause, the results surprised many.

“I didn’t expect this to happen, to be honest with you,” said Rep. Scott Taylor (R-Va.), a former Navy SEAL, who also backed the amendment during debate.

Stewart also said that he was shocked the amendment passed, but “that gives me hope that we can engage other members on this issue. Let’s just have the debate and not drag this thing on for another 16 years.”

Of course, there are Republican lawmakers who are not happy with the provision.

Defense subcommittee Chairwoman Rep. Kay Granger Norvell (Kay) Kay GrangerBottom line Helping our seniors before it's too late House approves .3 trillion spending package for 2021 MORE (R-Texas) called the amendment “a deal breaker and would tie the hands of the U.S. to act unilaterally or with partner nations with regard to al Qaeda and ... affiliated terrorism.”

The Foreign Affairs Committee has already pushed back on the language, arguing it violates the House’s rules and suggesting it may be stripped from the bill.

And Rep. Adam Kinzinger (R-Ill.), a former Air Force pilot, said he was “shocked and deeply troubled” by the amendment.

The question now is which GOP side will prevail in keeping the language in the House defense bill.

“I think it’s going to get ruled out of order,” Spoehr predicted, before adding that the idea is likely to stay.

“We think somebody should have this discussion again, but I don’t think this bill is the right tactic,” he said. “We have a proven track record in setting up a deadline, we think there’s no way we can miss it, and then we always find ourselves right against the deadline. We create an artificial crisis."

Stewart said that the bill will be discussed by the House leadership when lawmakers return from the July 4 recess, but a date has not been set.

“We’re going to have to talk,” he said. “We just haven’t had the time to look at it. We will, I just don’t know when.”