President Trump’s latest ultimatum on the Iran nuclear deal has increased the pressure on congressional negotiators who are trying to write a bill to improve the controversial pact, lawmakers acknowledged after his Friday announcement.

“I think we need to pass something that will put our country in a better position as it relates to what I think is just a horrible deal,” Wisconsin Sen. Ron Johnson, a Republican member of the Foreign Relations Committee, told the Washington Examiner.

Trump pledged to renew nuclear-related sanctions on Iran in 120 days if Congress fails to adopt an aggressive array of provisions targeting the regime’s nuclear and ballistic missile programs. A firmer articulation of the warning he issued in October, it comes months into negotiations between Foreign Relations Committee leaders, which may not get close enough for Trump's liking.

“Members of Congress are negotiating, in good faith, modest and reasonable revisions to [the Iran Nuclear Agreement Review Act] that would allow him to be more comfortable with the agreement,” Sen. Chris Coons, D-Del., another member of the foreign policy panel, told Capitol Hill reporters on Thursday.

Trump eschewed a modest tone in his Friday message to Congress. “Any bill I sign must include four critical components,” he said.

Those provisions include the broader access for international monitors seeking to inspect Iran’s nuclear program, in perpetuity, and a mandate to impose “severe” sanctions on Iran’s ballistic missile program. “If Iran does not comply with any of these provisions, American nuclear sanctions would automatically resume,” he said.

Senate Foreign Relations Chairman Bob Corker maintained a united front with Trump on Friday.

“At the president’s request, I have been working with his national security team and a number of my Senate colleagues on both sides of the aisle on a way to address the flaws in the agreement without violating U.S. commitments,” the Tennessee Republican said. “We have made significant progress over the past few months, and as I told the president when he called earlier today, we will continue working hard to achieve our shared goal: a better deal for America that will stand the test of time and actually prevent a nuclear-armed Iran.”

But his Democratic counterpart on the committee expressed more alarm that Trump would undermine the talks.

“Instead of leading an international negotiation on the agreement himself, however, the president’s statement making threats and dictating final terms of potential negotiations with Congress and Europe makes it more challenging to achieve this objective,” said Sen. Ben Cardin, D-Md.

Johnson expressed hope that Trump’s aggressive rhetoric would strengthen their hand without leading to an ultimate withdrawal from the deal.

“I was very happy with his announcement. I'm very supportive of what I'm hearing in terms of the potential legislation here on the whole Iranian agreement,” Johnson told the Washington Examiner. “And I think you have to give President Trump's bluster a fair amount of credit in terms of, I think we have the possibility of actually improving our position as it relates to that horrible deal.”

But Johnson added that the improvements need to be made in a way that wouldn’t terminate the agreement entirely. That’s because, he explained, Iran has already gotten “everything they wanted” out of the deal, so they have less incentive to remain in compliance with the pact if Trump tries to impose a major change unilaterally.

“Ripping it up makes no sense whatsoever,” Johnson said. “You want to strengthen it without having anybody give Iran cover to not continue to comply, and I believe that is the whole purpose of this effort.”

As lawmakers attempt to thread that needle, they run the risk of appearing soft on Iran by comparison to Trump or other congressional foreign policy makers. That difficulty is heightened in the Senate, where Corker has to develop a bill that can win enough Democratic support to avoid a filibuster.

“The president's announcement undeniably puts pressure on the Senate to start moving, and Corker will be feeling that from both his left and right,” a source close to the congressional negotiations told the Washington Examiner. “But the bigger challenge is going to come from the House, where Republicans are coalescing around competing legislation that's backed by influential Iran hawks."

"The optics are going to be bad for Corker, because it'll look like he's carrying water for Senate Democrats, but it's not just the optics. It's also the mechanics of legislation," he added. "This thing is going to have to be conferenced, which means that at a minimum Senate Republicans will have to take two tough votes on a weak bill that enshrines the Iran deal.”

Johnson hopes that Democratic proponents of the nuclear will leap at anything that discourages Trump from renewing the sanctions that former President Barack Obama agreed to waive.

“If I were a Democrat senator who didn't vote for disapproval of this agreement [2014], I would be all over this legislation,” he said.