"We now have a majority of colleagues, Democratic and Republican, who will stand strong for the principle that we shouldn't be at war without a vote of Congress," Sen. Tim Kaine, D-Va., said.
Kaine is the chief sponsor of a resolution that would bar Trump from using military force against Iran unless Congress specifically votes to authorize such action. The Virginia Democrat said the legislation could come up for a Senate vote as early as next week.
The U.S. should not order American troops to risk their lives in the Middle East "unless we have the guts to have the debate and have the vote to say that a war is in the national interest," Kaine said.
Democrats have argued that Trump acted recklessly when he authorized a drone strike killing Iranian Gen. Qasem Soleimani - a controversial decision that has heightened U.S.-Iran tensions and sparked fears of a broader conflict in the Middle East.
Republicans hold a 53-seat majority, and most GOP senators are expected to oppose the resolution, arguing it's unnecessary and unconstitutional. But Kaine said four GOP senators have agreed to support his resolution: Sens. Rand Paul of Kentucky, Mike Lee of Utah, Susan Collins of Maine and Todd Young of Indiana.
Young had expressed concerns about an earlier version of Kaine's bill. But Kaine said he has revised it to win more GOP support, and a spokeswoman for Young, Amy Grappone, confirmed Tuesday that he will support the revised bill.
Senate Majority Leader Mitch McConnell, R-Ky., called the war powers measure a "blunt instrument" and suggested it could project American weakness at a critical moment.
"Consider what message the Senate should send to Iran and the world at the very moment that America's actions are challenging the calculus in Tehran for the better," McConnell said in a Senate floor speech on Monday. "We appear to have restored a measure of deterrence in the Middle East, so let's not screw it up."
The House of Representatives passed a similar resolution last week, but GOP leaders slammed it as a meaningless PR stunt. Unlike the Senate measure, the House bill does not carry the force of law; Trump can ignore it.
Kaine's version, by contrast, carries the force of law.
Lee said he decided to vote for Kaine's resolution last week after Trump administration officials delivered a classified briefing on the Soleimani strike that he called "insulting and demeaning." Lee said officials warned lawmakers against debating the merits of war with Iran, arguing it would "embolden" Tehran to see division in Congress.
The message in that closed-door briefing was that "we need to be good little boys and girls and not debate this in public," Lee said. "I find that absolutely insane. It's un-American, it's unconstitutional, and it's wrong."
Even if Kaine's resolution passes the Senate and the House, Trump is likely to veto it. There's little chance Kaine and his allies could get the two-thirds super-majority needed to override a veto.
But debate in Congress will likely ratchet up pressure on Trump to justify his decision. The president and his advisers have given conflicting accounts of the intelligence behind the decision.
AG Barr: 'The president clearly had the authority to act' in Soleimani strike
Secretary of State Mike Pompeo initially suggested Soleimani was planning an "imminent" attack on U.S. interests in the Middle East - and then later said he didn't know where or when that attack would unfold.
Trump said on Friday that Soleimani was planning an attack against four U.S. embassies. But on Sunday, Defense Secretary Mark Esper said he had not seen intelligence to support that assertion.
The question of whether a Soleimani-planned strike was "imminent" is crucial; if it wasn't, then Trump should have sought congressional approval before killing him, lawmakers and national security experts say.
The strike would be legal if Soleimani "was involved in mounting a military action that was imminent - that is, about to happen," and if by killing him the plot was foiled, said Richard Hass, president of the Council on Foreign Relations and former diplomat during the George W. Bush administration.
But if those criteria were not met, then "what took place will be widely viewed as an action of choice and not necessity, one leading to an open-ended conflict between the United States and Iran," Hass told lawmakers at a House Foreign Affairs Committee hearing on Tuesday.
The chairman of that committee, Rep. Eliot Engel, D-N.Y., mocked the administration's shifting justifications for the attack.
"We heard the strike went forward because Soleimani did so many bad things in the past and was plotting for the future. Then, when that didn't work, they went back to an imminent threat, but we didn't know when or where it would take place," Engel said. "Next, it was going to be an embassy attack. Then four embassies were going to be attacked. Then maybe it wasn't four embassies."
Engel blasted Pompeo for declining to testify at Tuesday's hearing and said he will ask the Trump administration for its official legal justification for the strike.
"I think the administration is not being straight with the country or the Congress," Engel said.
Pompeo was traveling in California on Tuesday. He has staunchly defended the Soleimani strike as a legally appropriate action taken against a terrorist. He says Americans are safer as a result.
Contributing: William Cummings
This article originally appeared on USA TODAY: Iran: Senate could vote next week on limiting Trump's war powers