WASHINGTON Congressional efforts to limit President Donald Trump s authority to wage war unilaterally could prove more difficult than previous attempts despite greater fears of another war breaking out in the Middle East.

Sen. Tim Kaine (D-Va.) last week introduced a resolution to block Trump from further escalating hostilities with Iran after the president ordered a drone strike that killed its top general, Qassem Soleimani . Top Iranian leaders have vowed to take revenge for the strike; Trump, meanwhile, threatened to bomb the country if it retaliated.

The resolution does not prevent the U.S. from defending itself from imminent attack.

Kaines proposal is similar to an amendment he introduced last summer just as tensions between the two countries had begun to flare. It would have cut off federal funding of military action against Iran that was taken without congressional approval. During that vote, four Republicans broke with their party and voted with Democrats : Susan Collins of Maine, Rand Paul of Kentucky, Mike Lee of Utah and Jerry Moran of Kansas. The Senate ultimately rejected it, however, because it failed to garner the necessary 60 votes for passage.

Its not clear whether a proposal on limiting hostilities with Iran will get the same level of support this time, however. Unlike last year, it needs only a simple majority of 51 votes to pass. But, so far, only one of those four GOP senators Paul has publicly expressed interest in supporting Kaines measure.

Lee told reporters on Tuesday that he had concerns about portions of Kaines resolution laying out the congressional findings of fact introductory language detailing the situation in Iran and the authority delegated by Congress to the executive branch to wage war.

Moran said he wanted to hear a little bit more about the proposal before weighing in.

Collins said she planned to look at Kaines resolution but argued that it was important he not infringe on the presidents role as commander in chief to respond to an imminent threat.

This is one where the language really matters, she added on Monday.

Republican senators also expressed concerns about trying to limit the presidents war-making capabilities in the middle of a tense standoff with another country that has vowed to retaliate against the U.S., possibly against military and diplomatic outposts in the Middle East.

This is a really bad time to be doing it right now when we have what we have in front of us.... at this moment we shouldnt be talking about weakness and pulling things back, Sen. Jim Risch (R-Idaho), who chairs the Senate Foreign Relations Committee, told reporters on Tuesday.

The Senate also voted in 2018 to end U.S. support for a Saudi Arabian military campaign in Yemen that has been blamed for tens of thousands of deaths and mass starvation. During that vote, seven Republicans joined Democrats: the four mentioned above, Lisa Murkowski of Alaska and Sen. Todd Young. Trump vetoed the measure.

Kaine said Tuesday that he was willing to discuss concerns any senators may have with the resolution and argued that at the very least it offered a chance to put members of Congress on record about their role amid the conflict.

We owe it to our service members to have a debate and vote about whether or not its in our national interest to engage in another unnecessary war in the Middle East, he said in a statement.

The House is also expected to debate its own resolution trying to limit hostilities with Iran this week. That measure, which is similar to Kaines, is expected to pass in the lower chamber.