“Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not,” Trump tweeted Wednesday after Romney wrote an opinion column in the Washington Post calling the president’s character shortfalls “glaring” and saying his “words and actions have caused dismay around the world.”

Trump wrote that he would “much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!”

Romney, the 2012 Republican presidential nominee and former Massachusetts governor, wrote Tuesday that the president has “not risen to the mantle of the office,” distancing himself from Trump just two days before he will be sworn into the Senate. Romney won the Utah seat long held by GOP Senator Orrin Hatch in the November midterm elections.

Romney’s pointed criticism of the president suggests he may seek a role as a moral center for the GOP, after the death of Arizona Republican Senator John McCain left a vacuum. Two of the GOP’s most outspoken Senate critics of Trump—Flake of Arizona and Bob Corker of Tennessee—will also leave the Senate when the new congressional session begins Thursday after having decided not to run in the likelihood they would face tough primary challenges from Trump loyalists.

Romney’s Role

If Romney seeks the role of a critic, he may find himself alone like Flake and Corker, who tried to take on Trump over issues including his tariffs against China and allies like Canada. Other Republicans haven’t been willing to go along, as Trump continues to have a passionate and vocal base of support among a significant segment of GOP voters. But Romney’s stature in the party and in his home state may give him wider latitude to act as a counterweight to Trump in the party.

In the op-ed, Romney said many of Trump’s policies—on taxes, deregulation, confronting China on trade, and appointing conservative judges—have been longtime Republican goals. But he said setting policy is only part of a presidency.

“A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect,” Romney wrote. “As a nation, we have been blessed with presidents who have called on the greatness of the American spirit. With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable. And it is in this province where the incumbent’s shortfall has been most glaring.”

He also blasted Trump actions he said undermined U.S. leadership and alliances around the world.

“The world needs American leadership, and it is in America’s interest to provide it,” he wrote. “A world led by authoritarian regimes is a world — and an America—with less prosperity, less freedom, less peace.”

Although Trump endorsed Romney in the 2012 presidential campaign, the relationship took a more bitter direction in 2016. Romney emerged as a leader of the GOP “Never Trump” movement, attacking Trump as a “phony” and urging primary voters to pick someone else. Trump retorted by calling Romney “a choke artist” for losing to Barack Obama.

Trump took to Twitter (TWTR) on Wednesday to raise the specter of departing Sen. Jeff Flake, a moderate Republican from Arizona who often criticized the president’s stances. “Here we go,” the president said, noting how fast Romney’s position came.

“Question will be, is he a Flake? I hope not,” Trump said. “Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!”

Tensions between the two seemed to relax after the election. Trump even considered Romney for secretary of state, but chose Rex Tillerson instead.

Romney made clear in the op-ed that he largely plans to go along with the Republican Party—and not seek to challenge Trump at every turn.

“I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not,” he wrote. “I do not intend to comment on every tweet or fault.”