The US House of Representatives is set to vote on a resolution that aims to limit President Donald Trump's ability to make war on Iran.
The largely symbolic measure seeks to mandate congressional approval for any conflict with Iran, except in cases of an imminent attack against the US.
It is expected to pass the Democratic-held House, but faces tougher prospects in the Republican-controlled Senate.
Neither the US nor Iran have declared plans for further military action.
Iran this week fired missiles at Iraqi bases housing American forces, injuring no-one, after the US last week killed a senior Iranian commander in a Baghdad drone strike.
Thursday's measure directs the president to "terminate the use of United States Armed Forces" against Iran unless granted congressional authorisation.
It offers an exception when necessary to "defend against an imminent armed attack".
Even if the House measure clears Congress, it would not face a potential Trump veto because it is known as a concurrent resolution, which does not require a presidential signature.
The proposal cites the 1973 War Powers Act, which granted Congress the ability to check the president's power to commit the US to armed conflict.
But legal questions remain unresolved as to whether Congress can use a concurrent resolution to bind the president.
House Speaker Nancy Pelosi said on Thursday she did not believe Mr Trump had made the US safer after last week's drone strike that killed Iranian commander Qasem Soleimani.
Mr Trump, a Republican, meanwhile tweeted that he hoped "all House Republicans will vote against Crazy Nancy Pelosi's War Powers Resolution".
He also made a new claim about the intelligence behind the air raid, telling reporters at the White House the Iranians were "looking to blow up our embassy" in Iraq.