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.”