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 measure clears Congress, it would not reach the president's desk for a potential veto because it is known as a concurrent resolution, which lacks the force of law.
The measure 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.
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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.
The war powers resolution gathered momentum after a congressional briefing on Wednesday by administration officials seeking to justify the attack.
Following the briefing by the secretary of state, defence secretary and CIA director, two Republicans senators broke ranks.
Mike Lee of Utah and Rand Paul of Kentucky said they might back a similar resolution in the Senate seeking to limit the president's war powers.
Their potential defection raises the chances for the measure in the upper chamber, where Republicans hold a 53-47 majority.
