U.S. lawmakers are expected to vote this week on whether to order an end to military strikes against Iran. But even as President Donald Trump’s objectives and rationale for the war keep shifting, most Republicans, who hold a majority in Congress, are expected to back him.

The war on Iran was not authorized by Congress and is likely illegal under international law, which only permits wars that have been authorized by the U.N. Security Council or undertaken in self-defense. Trump and other administration officials have described the assault as preemptive.

U.S. lawmakers are expected to vote this week on whether to order an end to military strikes against Iran. But even as President Donald Trump’s objectives and rationale for the war keep shifting, most Republicans, who hold a majority in Congress, are expected to back him.

The war on Iran was not authorized by Congress and is likely illegal under international law, which only permits wars that have been authorized by the U.N. Security Council or undertaken in self-defense. Trump and other administration officials have described the assault as preemptive.

“It was my opinion that they [the Iranians] were going to attack first. They were going to attack. If we didn’t do it, they were going to attack first—I felt strongly about that,” Trump said on Tuesday from the White House, when asked if Israel forced his hand in the decision to go to war. “So, if anything, I might have forced Israel’s hand.”

But just yesterday, Secretary of State Marco Rubio described the decision somewhat differently.

“It was abundantly clear that if Iran came under attack by anyone, the United States or Israel or anyone, they were going to respond and respond against the United States,” Rubio told reporters. “The orders had been delegated down to the field commanders. It was automatic.”

The shifting messaging is unlikely to break the Republican Party’s near unity when it comes to backing Trump. But the votes expected this week, brought by Democrats in the Senate and House to order an end to the war under the 1973 War Powers Resolution, are likely to be exceedingly close given the tight margins in both two chambers. Some cross-party voting is likely on both sides.

Following an all-member classified briefing on Tuesday on the Iran war, the Senate will vote on Wednesday on an expedited bipartisan resolution from Democratic Sen. Tim Kaine and Republican Sen. Rand Paul. The House is likely to follow on Thursday with a vote on a similar measure from Democratic Rep. Ro Khanna and Republican Rep. Thomas Massie.

Democrats are arguing that the attacks on Iran, which have already included more than 1,700 strikes, are neither legal nor in the United States’ best interests and have accused Trump of launching a war of choice in violation of Congress’s constitutional responsibility to decide when the country goes to war.

“There was no imminent threat to the United States of America by the Iranians. There was a threat to Israel,” said Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, following a high-level classified briefing for congressional national security leaders on Monday. If “we equate the threat to Israel as the equivalent to an imminent threat to the United States, then we are in uncharted territory,” he added.

Republicans insist that the war is legal based on Trump’s inherent constitutional authority as commander in chief to defend the United States against imminent threats. But the administration has yet to describe the nature of the threat or provide details.

“Israel faced what they deemed to be an existential threat. Iran was building missiles at a …rapid clip,” Republican House Speaker Mike Johnson told reporters following the Monday briefing. “Because Israel was determined to act, with or without the U.S., our commander in chief and the administration … determined, because of the exquisite intelligence that we had, that if Israel fired upon Iran and took action against Iran to take out the missiles, then they would have immediately retaliated against U.S. personnel.”

Democrats are also criticizing Trump for not making the case for war to the American public, including during his annual State of the Union address to Congress last week.

“We have seen the goals for this operation change now, I believe, four or five times,” Warner said, noting that between taking out Iran’s nuclear capacity, destroying its ballistic missiles, bringing about regime change, and eliminating the country’s navy.

That failure to prepare Americans for the war means that many are now stranded in the region. By the time the State Department urged Americans to leave the area  on Monday, citing “serious safety risks,” thousands of regional flights had been canceled due to cross-border fighting. Fourteen countries were included in the statement, among them Israel, Egypt, and the United Arab Emirates. Iran has launched retaliatory strikes on several U.S. allies in the region, including those that host U.S. military bases.

“We’ve been talking to constituents all day today who are stuck in the region,” Republican Sen. Josh Hawley said on Monday. “So obviously, this is very, very dangerous.” Though Hawley in January cast a procedural vote in favor of a Democratic-led measure to terminate Trump’s U.S. military operation against Venezuela, he said he would not be voting for Kaine’s Iran war powers measure.

If Democrats lose this week’s Senate and House war powers votes, they might get another chance to try to constrain Trump through their power of the purse.

Johnson said that the potential for an emergency Iran-related military funding bill had already been discussed among congressional appropriators, but precisely what funding levels would be included in the bill depended on the length of U.S. military operations, which isn’t currently known.

Trump and his senior cabinet officials have offered differing assessments of how long the war might last, while also warning that the U.S. military was still in ramp-up mode in terms of the deployment of forces.

If combat operations against Iran are still ongoing two months from now, Trump will run afoul of the War Powers Act, which states that military operations not authorized by Congress must end within 60 days of their initiation.

Running out the clock on unauthorized operations would likely anger some Republicans as the midterm election gets closer. So would mounting U.S. war casualties or the deployment of ground troops to Iran.

“We should not be going into a war in the Middle East or anywhere without good, solid reason that has had its tires kicked,” Kaine said in a Monday floor speech. “This president has asserted no real reason that is clear to the American public, and that’s why the American public so far is so against this.”