Senate Republicans are on Wednesday expected to vote down a Democratic-backed war powers resolution that would prevent Donald Trump from continuing the conflict against Iran, with majority leader John Thune arguing the president is “acting in the best interest of the nation”.

Democrats have condemned Trump for ordering an air campaign against Iran without first seeking permission from Congress, while offering shifting explanations of its objectives. The war powers resolution introduced by Democratic senators Tim Kaine, Adam Schiff and minority leader Chuck Schumer would force an end to US participation in the current hostilities and require the president to go to Congress before re-entering the war.

“We shouldn’t be at war without a debate and vote. That was what the framers intended,” Kaine told reporters Tuesday. “We protect our troops when we do it the right way. We put them at risk when we do it the wrong way.”

The resolution will require 50 votes to advance. Democrats control 47 seats, but the Pennsylvania senator John Fetterman said he will oppose the resolution, meaning they’ll need at least five Republicans to join with them for it to succeed.

The odds of that happening dimmed on Tuesday when Thune told reporters he believes Trump has the proper authorization for the campaign, which the US military is carrying out alongside Israel.

“The president has the authority that he needs to conduct the activities, the operations that are currently under way there,” Thune replied at a Tuesday press conference, when asked if Trump should at some point seek congressional approval to continue the conflict.

“I think the president is acting in the best interest of the nation and our national security interests by ensuring that he’s protecting Americans and American bases and installations in that region, as well as those of our allies.”

The Democratic senator Chris Murphy predicted that the resolution “will likely fail”, but said it would nonetheless offer an important opportunity to consider the justification for the conflict.

“We shouldn’t be voting to proceed to other pieces of legislation until we get a debate on this deeply unpopular, immoral and illegal war with Iran,” Murphy told reporters after receiving a classified briefing from administration officials Tuesday afternoon.

In the House of Representatives, Democrat Ro Khanna and Republican Thomas Massie have introduced their own war powers resolution, which is expected to come up for a vote on Thursday. But that also faces Republican opposition, with speaker Mike Johnson saying on Monday that it would be “dangerous” to force an end to US involvement in the conflict.

“The idea that we would take the ability of our commander in chief, the president, take his authority away right now to finish this job, is a frightening prospect to me. It’s dangerous,” Johnson said. “I am certainly hopeful and I believe we do have votes to put it down.”

Even if a war powers resolution was to clear the House and Senate, Trump could veto it, and lawmakers would need two-thirds majorities in both chambers to override that. The president has also retaliated against lawmakers who have broken with him over foreign policy issues, including saying that four Republican senators who voted in January to advance a war powers resolution blocking hostilities against Venezuela “should never be elected to office again”. Two of those senators later dropped their support, and the measure failed.

This week’s votes may nonetheless offer some Trump allies the opportunity to come out against a conflict that has thus far resulted in the deaths of six US service members, as well as 787 people in Iran, according the country’s Red Crescent.

“The constitution conferred the power to declare or initiate war to Congress for a reason, to make war less likely,” the Kentucky senator Rand Paul wrote on X as fighting began over the weekend. “As with all war, my first and purest instinct is [to] wish Americans soldiers safety and success in their mission. But my oath of office is to the constitution, so with studied care, I must oppose another Presidential war.”

The right wing congressman Warren Davidson was similarly sceptical of involving the US in the conflict without congressional authorization, telling CNN in an interview that Trump “campaigned … against starting a war in Iran”.

But he said he hoped that administration officials who were set to brief lawmakers on Tuesday afternoon would be able to convince him of the president’s rationale.

“I want to see the intelligence that president Trump found persuasive, and, you know, persuade us. I’m open to being persuaded,” Davidson told CNN.