Just days before becoming Utah’s next senator, Mitt Romney denounced President Donald Trump’s performance in a Washington Post op-ed, arguing the president has failed to inspire or conduct himself with honesty and integrity.

And he says Trump has damaged U.S. relationships with trusted allies at a fraught time.

“On balance, his conduct over the past two years, particularly his actions this last month, is evidence that the president has not risen to the mantle of the office,” Romney wrote in the damning opinion piece that praised past 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,” he wrote. “And it is in this province where the incumbent’s shortfall has been most glaring.”

The critique laments the recent ousters of Defense Secretary Jim Mattis and chief of staff John Kelly and the president’s recent assertion that the U.S. has been a “sucker” by taking a leadership role in international institutions or foreign conflicts, such as in Syria.

Trump took to Twitter early Wednesday saying, “Here we go with Mitt Romney, but so fast!” He added, “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!”

Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not. 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! — Donald J. Trump (@realDonaldTrump) January 2, 2019

Romney’s criticism harks back to his speech in March 2016, before Trump won the Republican nomination.

In that address at the University of Utah, Romney called Trump a “fraud” and a “phony,” warning about his temperament and his foreign policy positions.

Just how Romney, the 2012 Republican presidential nominee, would respond to and interact with Trump has been a raging debate ever since Romney announced his intention to seek the seat held by retiring Sen. Orrin Hatch, who is one of Trump’s biggest supporters in the Senate. Adding to the suspense are the departures of Republican senators, such as Jeff Flake of Arizona and Bob Corker of Tennessee, who have been the party’s counterpoint to the president.

Romney easily won the Senate race in Utah, his adopted state and a Republican stronghold that has long been wary of Trump. Like all new members of Congress, Romney will take the oath of office Thursday.

An Associated Press poll released in late November found that 64 percent of Utah voters and half of Romney’s supporters wanted to see him stand up to the president.

In the op-ed published Tuesday evening, Romney included the stance he repeated often as he campaigned in Utah. That he would support the policies he believes are best for Utah and the nation, and speak up on “significant statements or actions” that he finds “divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.”

He also highlighted areas of agreement with the president, including the tax plan, the appointment of conservative judges, a recent criminal justice reform package and fighting China over trade practices.

But at no point in Romney’s Senate run did he level such broad criticism as he did in this column, saying “Trump’s words and actions have caused dismay around the world,” empowering China and Russia.

He wrote “to reassume our leadership in world politics, we must repair failings in our politics at home. That project begins, of course, with the highest office once again acting to inspire and unite us. It includes political parties promoting policies that strengthen us rather than promote tribalism by exploiting fear and resentment. Our leaders must defend our vital institutions despite their inevitable failings: a free press, the rule of law, strong churches, and responsible corporations and unions.”

Jason Perry, the director of the U.'s Hinckley Institute of Politics, said Romney’s op-ed shows that Utah’s freshman senator will be “a powerful voice" with national stature.