In this corner are two wealthy businessmen, Donald Trump and Wilbur Ross. In the opposing corner are Speaker Paul D. Ryan, Senate Finance Committee Chairman Orrin G. Hatch and just about the entire Republican conference.

Not long ago, Trump boasted of leading the most unified Republican Party in American history. A few weeks later, his talk of tariffs on imported steel and aluminum and declaration that “trade wars are good” have caused this marriage of convenience to sour.

The president’s takeover of the Republican Party was rocky at first, with GOP lawmakers forced to distance themselves from their previous harsh words about Trump. But they soon bonded over eviscerating as much of former President Barack Obama’s legacy as they could, nixing federal regulations, trying to take down the Affordable Care Act, putting conservative judges on courts across the country and slashing tax rates.

Five days before Christmas, Republican lawmakers smiled widely and snapped mobile phone pictures of Trump on the White House’s South Portico. They were there to celebrate with the president — both chambers had passed a GOP-crafted bill that Trump would sign two days later cutting some tax rates and altering the federal code. Since then, however, the relationship has again turned rocky.

[Trump Attacks Canada Over ‘Highly Restrictive’ Trade Tactics]

The president frustrated some Republican members when he and his staff torpedoed a bipartisan Senate immigration bill and when he sided with Democratic ideas most of the time in a nationally televised White House meeting on gun violence. But he really angered many of them on Thursday when he announced — taking his staff and administration by surprise — he will impose tariffs on imported steel and aluminum.

GOP members, especially top leaders, have largely avoided publicly criticizing the president. Even more rare have been outright expressions of opposition. But the tariff plan, something the pro-trade party has long opposed, was a bridge too far in Trump’s freewheeling “America first” presidency.

And some Republicans are not ruling out some kind of action to block or blunt Trump’s promised 25 percent steel and 10 percent aluminum tariffs.

“Congressional leaders won’t rule out potential action down the line,” said one GOP source.

Hatch plans to send Trump a letter this week, and intends to follow up with a phone call to the president, an aide to the Utah Republican said Monday. 

If the tariffs go into effect, “American manufacturers, businesses, and consumers would be forced to bear the brunt paying more for steel and steel products,” the aide said. “Such action could very well undercut the benefits of the pro-growth tax reform we fought to get on the books.”

Full speed ahead

For his part, the president signaled Monday he is moving ahead with plans to formally roll out and implement the tariffs this week.

“No, we’re not backing down,” Trump said when asked about Ryan’s opposition. “Right now, 100 percent.”

The White House downplayed the notion of tension, with Press Secretary Sarah Huckabee Sanders saying the president has had and intends to maintain a “great relationship with Speaker Ryan.” Still, “that doesn’t mean we have to agree on everything,” she said, adding voters knew Trump’s tough trade stances when they elected him.

The source’s threat came shortly after Ryan, House Ways and Means Chairman Kevin Brady and Ways and Means Trade Subcommittee Chairman Dave Reichert added their names to the list of GOP members pushing the GOP president to reconsider.

Brady and Reichert have drafted a letter to Trump “expressing concerns about ‘the prospect of broad, global tariffs on aluminum and steel imports’ — and are seeking signatures from Committee Republicans,” Ways and Means spokeswoman Lauren Aronson said. The lawmakers called on Trump to work with lawmakers on “trade policies that build off the momentum of the president’s tax cuts.”

Brady confirmed that he and Reichert will encourage Trump to “tailor his actions.” 

“My view is the president should exempt all fairly traded steel and aluminum,” the Texas Republican said Monday.

When asked if Congress could pursue legislation to block the tariffs if Trump does not tailor them on his own, Brady said,“The focus right now ought to be on the actions he takes this week and the actions he takes going forward … as the impact from our trading partners starts to occur.”

Class is in session

In recent days, GOP members have essentially been delivering a collective and ongoing economics lecture to Trump that summarizes the party’s long-held opposition to tariffs.

“Tariffs should be narrow, targeted and focused on addressing unfairly traded products, without disrupting the flow of fairly traded products for American businesses and consumers,” Aronson said, while Brady said blanket tariffs could harm energy and manufacturing industries in Texas. 

Ryan has frustrated many on the political left by choosing to remain silent or defend some of Trump’s more outlandish statements. But he did not mince words, via a spokeswoman, on Monday about his opposition to the plan.

“We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan,” Ryan spokeswoman AshLee Strong said in a statement. “The new tax reform law has boosted the economy and we certainly don’t want to jeopardize those gains.”

As speaker, Ryan is second in line to the presidency, after the vice president, under the Constitution. He is the highest-ranking elected Republican or party official to voice his criticism of the president’s tough tariff talk.

[Even on North Korea, Trump Shifts Wildly]

Ryan’s announcement sent ripple waves across Washington. That’s because before Monday, GOP opposition to Trump’s trade haymaker was mostly limited to rank-and-file Republican members like Republican Sen. Ben Sasse of Nebraska and chairmen like Brady and Hatch.

“Whoever advised him on this ought to be reprimanded. In all honestly it’s not going to help America,” Hatch said last week. “It’s going to cause higher prices around here and it’s going to make us less competitive.”

“Trade wars are never won. Trade wars are lost by both sides,” Sasse said in a Friday statement. “Kooky 18th century protectionism will jack up prices on American families — and will prompt retaliation from other countries. Make no mistake: If the President goes through with this, it will kill American jobs — that’s what every trade war ultimately does. So much losing.”

But Trump — despite dismissing the advice of economists and his top economic adviser, Gary Cohn — contends his party mates are wrong to predict the import fees will not trigger global chaos.

“I don’t think so,” he told reporters on Monday. “I don’t think we’re going to have a trade war.”

But because Trump increasingly swings wildly between policy stances, will it even come to that? What if he simply decides to never impose the tariffs?

“If he says something different, it’ll be something different,” Ross said on “Meet the Press” on Sunday. “I have no reason to think he’s going to change.”

Lindsey McPherson contributed to this report.