The U.S. Senate has passed a bipartisan resolution curbing President Donald Trump's authority to launch military operations against Iran.
The measure, authored by Senator Tim Kaine (Democrat-Virginia) says Trump must win approval from Congress before engaging in further military action against Iran.
Eight Republicans joined with Democrats to pass the resolution by a 55-45 vote.
Kaine and other supporters said the resolution was not directed at Trump, but instead was an important reassertion of congressional power to declare war.
Under the U.S. Constitution, Congress has the power to declare war -- not the president.
Trump and his supporters say the measure sends the wrong signal to Iran and other potential adversaries.
"It is very important for our Country's SECURITY that the United States Senate not vote for the Iran War Powers Resolution. We are doing very well with Iran and this is not the time to show weakness," the president tweeted.
Trump is likely to veto the measure, having dismissed limiting the president's war powers as a sign of "weakness."
The president recently used his veto on a War Powers Resolution on Yemen.
The Democratic-controlled House of Representatives passed a separate, nonbinding war-powers resolution last month, as Democrats and some Republicans fumed over Trump's failure to fully inform them about his Iran strategy.
The move came after Trump ordered a drone strike that killed Major General Qasem Soleimani, the leader of Iran's elite Quds Force, outside Baghdad.
He did not inform Congress until after the strike.
In response to Soleimani's killing, Tehran launched missile strikes on two bases hosting U.S. troops in Iraq.
The Pentagon says 109 U.S. service members have been diagnosed with mild traumatic brain injury following the attack on the Ain al-Asad Air Base in western Iraq, though no one was killed.
The House could take up the Senate resolution later this month.