"We now have the 51 votes that we need for the version that's the bipartisan version," said Democratic Senator Tim Kaine, who introduced the measure intended to force Trump to seek authority from Congress if military hostilities with Iran last for more than 30 days.

Kaine named four Republican senators -- Rand Paul, Mike Lee, Todd Young and Susan Collins -- who he said will join the 47 Democrats voting in favor in the 100-member chamber.

Collins said in a statement that she would co-sponsor Kaine's revised resolution because it "reasserts Congress's constitutional role and recognizes that the framers (of the US Constitution) did not vest in the president the authority to declare war unilaterally."

The measure is privileged, meaning Republicans who oppose it would not be able to block a vote once the legislation "ripens."

That process occurs Sunday, and the resolution would first be available for Senate consideration next week, Kaine said.

But the measure could bump up against Senate responsibilities that will take center stage in coming days: Trump's impeachment trial, which could begin next Tuesday.

"We will work out the timing," Senate Minority Leader Chuck Schumer said of the war powers resolution.

"We have to figure out how it intersects with impeachment, but we believe that this resolution is the right way to go."

Should the measure pass the Senate it would serve as a congressional rebuke of Trump after his order to kill an Iranian commander triggered retaliatory missile strikes by Tehran and dramatically escalated tensions.

The House passed its own war powers resolution last week. But unlike the Senate version, the House's measure is not binding, meaning it would need to pass the Senate version in order to get it to the president's desk.

Trump would almost certainly veto a measure that clips his powers to take military action, and there likely are not sufficient votes in Congress to overcome a veto.

Kaine said he was meeting with more Republicans to broaden support for his effort, but one Republican who has been an occasional Trump critic was not on board.

Senator Mitt Romney said he will vote no because he does not want to "tie the president's hands" in responding to further Iranian aggression.