Sen. Bob Corker (R-Tenn). (J. Scott Applewhite/AP)

The Republican plan to restrain Iran's nuclear ambitions without blowing up the international deal on the issue depends on how deftly President Trump's most vocal GOP critic can operate under a new ultimatum: Deliver changes, or Trump will rip up the nuclear deal on his own.

It was not quite a week ago that Sen. Bob Corker (R-Tenn.) excoriated Trump in an interview with the New York Times, warning that the president was steering the country toward "World War III" and tweeting that the White House had become an "adult day-care center," in response to the president's Twitter tirade against him. Corker's comments came just days after he told reporters that members of Trump's national security team were the only thing keeping the country from "chaos."

In an interview Friday evening, Corker summed up the president's Iran speech as a case in which "his advisers have prevailed" and the president "stayed on script."

But the highest praise he had to offer the president for his speech — and in particular, Trump's threat to Congress that he would act to undermine the deal — was a reserved "fine."

"I'm not going to answer you on any of the negative stuff today," he said. "This is, to me, a good day. It's a good day for the country."

As the chairman of the Senate Foreign Relations Committee, Corker has the job of maneuvering through the Senate the Iran legislation he spent eight months hammering out with the White House, the State Department and Republican hawk Sen. Tom Cotton (Ark.). Their plan: to freeze Iran's current "breakout" window for a nuclear weapon at one year, and automatically reimpose economic sanctions if Iran were to narrow that window.

The approach of the plan Corker and Cotton designed is threefold: give the International Atomic Energy Agency (IAEA) more authority to inspect Iranian sites for compliance than the nuclear deal currently allows, restrict Iran's development and use of centrifuges that could be used to enrich uranium to weapons grade for a nuclear bomb, and make sure that Iran does not develop the international ballistic-missile capacity to deliver a nuclear weapon.

It took until just over a month ago to get Trump's national security team on board with those specifics, Corker said. But he would not say when or if ever the president got on board with the plan. Even on Friday morning, Corker and Trump seemed anything but unified in their approach.

In a call with reporters barely an hour before Trump's planned announcement that he was going to "decertify" the Iran accord and ask Congress to toughen it, Corker said repeatedly that he could not say whether the president would stick to the script. And he warned that he would not by himself shoulder the burden of selling Democrats on the proposed changes to the policy on Iran.

Securing the support of Democrats would be impossible without the European parties to the Iran nuclear pact being on board, Corker told reporters, adding that "it's up to the administration to be able to bring our European allies along with us."

He stressed again in an interview that securing European support for the changes was far more important than any threats or ultimatums to obtain Democratic support.

But during his speech, Trump did the opposite of promising to pull his weight to see the deal through. Instead, he issued a threat, saying that if Congress and the European countries couldn't come up with a plan to further restrain Iran, "then the [Iran nuclear] agreement will be terminated . . . our participation can be canceled by me, as president, at any time."

Procedurally, Corker has thus far been effectively ignoring the president's announcement where he can. He refused to pursue the 60-day, expedited schedule for congressional action after the president's decertification announcement, opting instead to hold committee hearings and work with Senate Majority Leader Mitch McConnell (R-Ky.) to find Senate floor time before the end of this year.

Corker hinted Friday that Trump's national security team would have preferred that Trump ask Congress to address Iran's nuclear ambitions without refusing to certify Iran's compliance with the deal, noting that "there was a lot of discussion" on that point. Some, such as the chairman of the Joint Chiefs of Staff, Gen. Joseph F. Dunford Jr., said publicly last week that members of Trump's national security team believed Iran was in compliance with the deal.

"There was just not a way to get there," Corker said in an interview. "To do that would have taken the Democrats wanting to weigh in on the front end, and that was not a place they were willing to go last week."

For days, Democrats refused to discuss the rumored details of Republican proposals, instead raising a general outcry against Trump's planned decertification announcement. Only the Senate Foreign Relations Committee's ranking Democrat, Benjamin L. Cardin (Md.), and later, Senate Minority Leader Charles E. Schumer (D-N.Y.) were directly informed of details of the GOP plan before Trump's announcement, and neither had seen the proposed legislation.

Democrats now argue that parts of the GOP proposal may violate the nuclear pact, and they are promising to hold firm against anything that might be perceived as a violation of it.

"I think it's going to be a challenge to get anything past the United States Senate on this subject," Cardin said. "You've got people who basically want to walk away from the agreement and people who don't want to do anything that could be interpreted as walking away. So it's difficult to find that sweet spot."

This is also politically unfamiliar territory for Corker.

The senator, who is retiring when his term ends next year, is used to working with Democrats on Iran policy. He and Cardin together drafted the bill that gave Congress a chance to block the Iran deal; he and Cardin teamed up again, after several months of negotiating, to write legislation passed in the summer that severely stepped up sanctions against Iran over its ballistic-missile tests and international deployment of the Iranian Revolutionary Guard Corps.

The current plan, as outlined by Corker, seeks to give the IAEA more power to verify Iran's compliance than it has under the nuclear deal and to further restrict Iran's development and employment of centrifuges that could be used to enrich fuel for a nuclear weapon. The purpose of the plan is to ensure that in the long term, Iran is never less than one year away from having a nuclear weapon, which is what experts estimate to be its current capacity. If Iran were to narrow that window, the plan would require that sanctions automatically be put in place.

But Democrats are not convinced that they can safely tackle those "gaps" in the nuclear deal without the United States reopening negotiations with Tehran or violating the deal, Cardin said.

He also questioned whether congressional Republicans, or Trump, would be able to convince Britain, France, Germany and the European Union to agree to the proposals.

"In my conversations with the Europeans, they're not there," Cardin said.

Democrats argued all week that Trump's clear hatred of the Iran deal makes it difficult to talk about making changes, because anything approaching a change to the deal will be seen as a surreptitious effort to discredit it. His threat Friday to rip it up if Republican plans do not prevail, some said, further complicates how any legislation will be perceived.

"Talking tough and saber-rattling may be politically expedient for President Trump today, but Congress needs to take a more responsible approach," said the Senate Armed Services Committee's ranking Democrat, Jack Reed (R.I.).

"Iran is living up to its commitments," said Sen. Dianne Feinstein (D-Calif.), urging lawmakers to "vote against reimposing sanctions" lest "the United States, not Iran" be at fault for reneging on its commitments under the deal.

Corker put the chances of getting a bill through the Senate at "reasonable." He acknowledged, however, that Democrats have little incentive to trust the president without some guarantee that Trump would not be able to unpredictably rip up the deal on his own.

"I have a feeling that before this is all done, the other side of the aisle is going to want some type of assurances" that Trump will not rip up the deal, Corker said Friday, suggesting those assurances might have to be written into law. The Corker-Cotton proposal also proposes changes to the frequency and type of certification reports the president makes to Congress about the Iran deal.

For Democrats, the challenge will be separating their considerable disdain for Trump and his decision not to certify the deal from what they see as the best interests of the country.

"We're more than willing to change the certification requirements. There's things we could look at," Cardin said, referring to a proposal to change the frequency and criteria for the president's reports to Congress about Iran's compliance.

"The bad guy here is Iran, so we've got to keep our eye on Iran," Cardin said. "We want to agree that it's in our interests to remain in the [deal], provided we strictly enforce Iran's obligations under it."

But, he added, "At this point, if nothing is done and the president takes no action, we're okay."