WASHINGTON (AP) — The Senate approved a bipartisan measure Thursday aimed at limiting President Donald Trump’s authority to launch military operations against Iran, with eight Republicans joining Democrats in a post-impeachment bid to constrain the White House.

The rebuke was the Senate’s first major vote since acquitting Trump on impeachment charges last week. Trump is expected to veto the war powers resolution if it reaches his desk, warning that if his “hands were tied, Iran would have a field day.’”

The measure, authored by Sen. Tim Kaine, D-Va., says Trump must win approval from Congress before engaging in further military action against Iran. Kaine and other supporters said the resolution, which passed 55-45, was not about Trump or even the presidency, but instead was an important reassertion of congressional power to declare war.

While Trump and other presidents “must always have the ability to defend the United States from imminent attack, the executive power to initiate war stops there,” Kaine said. “An offensive war requires a congressional debate and vote.”

The Senate vote continues a pattern in which Republican senators have shown a willingness to challenge Trump on foreign policy, a sharp departure from their strong support during impeachment and on domestic matters. Congress moved to impose restrictions on U.S. involvement with the Saudi-led war in Yemen last year after U.S.-based journalist Jamal Khashoggi was killed in a gruesome murder at Saudi Arabia’s consulate in Turkey.

The bipartisan vote was a rare exertion of authority from Congress, the first since passage of the War Powers Act of 1973. And Trump promptly vetoed it.

The Democratic-controlled House passed a separate, nonbinding war powers resolution on Iran last month. The House could take up the Senate resolution later this month, House leaders said. Two-thirds votes in the House and GOP-run Senate would be needed to override an expected Trump veto of the war powers resolution.

Answering a claim by some of Trump’s supporters and Trump himself that the measure would send a signal of weakness to Iran and other potential adversaries, Kaine said the opposite was true.

“When we stand up for the rule of law … and say ‘This decision is fundamental, and we have rules that we are going to follow so we can make a good decision,’ that’s a message of strength,” Kaine said. “If we’re to order our young men and women … to risk their lives in war, it should be on the basis of careful deliberation by the people’s elected legislature and not on the say-so of any one person.”

Sen. Mike Lee, R-Utah, agreed. Lee supports Trump’s foreign policy, including toward Iran, but said Congress cannot escape its constitutional responsibility to act on matters of war and peace.

As the Senate debate made clear, “there is abundant support for the United States taking tough positions with regard to Iran,” Lee said. ”And as part of that we want to make sure that any military action that needs to be authorized is in fact properly authorized by Congress. That doesn’t show weakness. That shows strength.”’

Trump disputed that, arguing on Twitter that a vote against Kaine’s proposal was important to national security and pointed to the Jan. 3 drone strike that killed Iran’s top general, Qassem Soleimani.

“We are doing very well with Iran and this is not the time to show weakness. Americans overwhelmingly support our attack on terrorist Soleimani,” Trump said. “If my hands were tied, Iran would have a field day. Sends a very bad signal. The Democrats are only doing this as an attempt to embarrass the Republican Party. Don’t let it happen!”

Tehran responded to the U.S. attack on Soleimani by launching missiles at two military bases in Iraq that house American troops. The attack caused traumatic brain injuries in at least 64 U.S. soldiers, the Pentagon said..

Democrats and Republicans alike criticized a briefing by the Trump administration shortly after the drone strike, saying U.S. officials offered vague information about a possible attack being planned by Iran but no substantial details.

Kaine has long pushed for action reasserting congressional power to declare war. At Republicans’ request, he removed initial language that targeted Trump in favor of a generalized statement declaring that Congress has the sole power to declare war. The resolution also directs Trump to terminate use of military force against Iran or any part of its government without approval from Congress.

Sen. Susan Collins, R-Maine, a co-sponsor, called the resolution “much needed and long overdue.” In recent decades, “Congress has too often abdicated its constitutional responsibility on authorizing the sustained use of military force,” she said.

Senate Majority Leader Mitch McConnell, R-Ky., and many other Republicans opposed the resolution, saying it would send the wrong message to U.S. allies. “Just as we have successfully sent Iran this strong signal of our strength and resolve (by killing that country’s top general), a blunt and clumsy war powers resolution would tie our own hands,” McConnell said.

The three senators seeking the Democratic nomination for president — Amy Klobuchar of Minnesota, Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts — all returned to the Capitol from campaigning and backed the war powers resolution.

Besides Collins and Lee, Republicans joining Democrats were Sens. Lamar Alexander of Tennessee, Bill Cassidy of Louisiana, Jerry Moran of Kansas, Lisa Murkowski of Alaska, Rand Paul of Kentucky and Todd Young of Indiana.