An urgent entreaty from two of President Trump’s top cabinet members to lawmakers weighing ending U.S. support for the Saudi Arabia-led coalition in Yemen has backfired, sparking a symbolic victory for critics of the policy and revealing a growing discomfort with the Trump administration’s embrace of Riyadh.

On Wednesday morning, Defense Secretary Jim Mattis and Secretary of State Mike Pompeo gave a classified briefing to senators. They warned that if the U.S. “disengages” from the conflict in Yemen, it could damage fragile peace negotiations “by breathing new life into the Houthis' combat operations, just when they are reluctantly engaging with the U.N. interlocutor,” Mattis said.

But senators from both parties emerged from the briefing unpersuaded. Immediately following the briefing, several lawmakers who had opposed a previous measure to curtail U.S. involvement announced that they would now support a similar measure. And four hours after the briefing ended, the Senate voted, 63-37, to formally open debate on a War Powers resolution on the U.S. involvement in Yemen. The measure would give the administration 30 days to end its military involvement, yet allow counterterror operations against al Qaeda to continue. It was a dramatic reversal of a similar vote held in March that failed, 55-44.

Members of both parties expressed frustration that CIA Director Gina Haspel had not come to brief them on the agency’s assessment of the Saudi crown prince’s involvement in the murder of the Washington Post columnist Jamal Khashoggi. Some lawmakers told reporters that Haspel was blocked from appearing by the White House; National Security Advisor John Bolton has denied this.

“I changed my mind, because I’m pissed,” said Sen. Lindsey Graham, R-S.C. Graham supports cutting U.S. support in the campaign but opposes the use of the War Powers Act to do so, and had been expected to vote against advancing the measure.

Sen. Cory Gardner, R-Colo., said before the vote that “there are certainly members who are weighing the absence of the CIA director in how they will vote.”

But the policy implications of Wednesday’s vote—the first procedural hurdle for the legislation—are difficult to judge. A War Powers resolution on Yemen is a nonstarter in the House in the waning days of this Congress, although a Democratic House could revive the issue in 2019.

Meanwhile, several powerful Republican senators who supported the procedural vote have signaled that they will seek to amend the underlying legislation during debate. Graham told Defense One that he would work to substitute his own legislation for the War Powers resolution on the floor; Graham’s bill, which also has Democratic co-sponsors, would suspend arms sales to Saudi Arabia and impose sanctions on those responsible for the Khashoggi murder, among other things.

Sen. Bob Corker, R-Tenn., the retiring chairman of the Senate Foreign Relations Committee, also told reporters that he was willing to support the procedural vote “knowing it can be amended.” If the Trump administration does not hold the Kingdom to account for Khashoggi’s murder, he said, “then there will be another point in time where we can decide whether we like the substance that may be created in an amendment process going through this.”

At issue is not an appetite to curtail assistance or hold Saudi Arabia accountable for the Khashoggi murder, but rather the use of the War Powers Act of 1973 to do so. Critics of the resolution argue that the kind of military support being provided to Saudi Arabia—including intelligence, targeting assistance and, until recently, aerial refueling—does not meet the legal threshold that would require the administration to seek a war authorization from Congress.

That’s how Graham sees it — as does the Trump administration.

Behind closed doors, Mattis and Pompeo warned lawmakers that withholding support would damage the nascent diplomatic negotiations led by UN Special Envoy Martin Griffiths, and could even increase civilian casualties by depriving Saudi Arabia of U.S. targeting expertise.

Mattis made the case briefly in his opening statement, calling it “a key point” that the U.S. is not “operationally involved in hostilities in Yemen’s civil war.”

“Our military efforts are in accordance with the War Powers Resolution's provision that U.S. forces do not ‘command, coordinate, participate in the movement of, or accompany regular or irregular military forces of any foreign country or government when such military forces are engaged, or there exists an imminent threat that such forces will become engaged, in hostilities,’” he said.

The White House went even farther in a Wednesday statement, arguing that the “fundamental premise” of the resolution is flawed because ”United States forces are not engaged in hostilities between the Saudi-led coalition and Houthi forces in Yemen.”

Sen. Chris Murphy, D-Conn., one of the authors of the legislation, dismissed the notion that the United States is not engaged in hostilities in Yemen.

“Nobody in that room is buying the idea that what is happening in Yemen today doesn’t qualify as hostilities under the War Powers Act,” he told Defense One. “They admit that we are helping them select targets. We may not give the order but we are actively involved in helping the Saudis pick targets.”

“That is the definition of hostilities: picking who you kill and who you don’t kill.”

Murphy also dismissed the notion that defining U.S. activities in Yemen as “hostilities” under the War Powers Act would create a de facto principle that could improperly constrain the president’s authority in other partnered operations.

“It’s certainly an interesting case because it does call into question other relationships we have and whether that’s hostilities or not, but you would take these up on a case-by-case basis,” he said. “There no automatic precedent that gets set if you call this ‘hostilities,’ there is no process by which the administration would be bound by this vote.”

No matter its fate, the procedural vote on Wednesday was historic. Critics of military support to the Kingdom have for years sought to get legislation to the floor to curtail American involvement in the Yemen conflict. Republican leadership in the House shot down a similar effort just weeks ago.

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The Trump administration, led by White House Senior Advisor and presidential son-in-law Jared Kushner, views Saudi Arabia as a key ally in its efforts to constrain Iran in Middle East and combat terror and has been reluctant to shift that relationship. In an extraordinary statement last week that sent shock waves rippling through Washington, the president reaffirmed the U.S. commitment to “standing with Saudi Arabia” even though “it could very well be that the Crown Prince had knowledge of” Khashoggi’s murder. The CIA, led by Haspel, has reportedly concluded that Crown Prince Mohammed bin Salman ordered the killing, fueling speculation on Capitol Hill about why she was apparently not allowed to appear. (Senate Minority Whip Dick Durbin, D-Ill. said that “the most persuasive presence at this briefing was an empty chair.”)

But the international firestorm over the grisly killing helped stoke simmering frustration with mounting civilian deaths in Yemen’s civil war. In August, a U.S.-purchased bomb dropped by the Saudi coalition killed 40 school children on a bus; in the end, it was Haspel’s failure to appear that seemed to shift the political winds.

“This is B.S.,” Graham told reporters. “I want the CIA to brief me.”