Mitt Romney slammed President Trump's leadership, writing in an editorial in The Washington Post that "the president has not risen to the mantle of the office." Romney will be sworn in as the junior senator from Utah on Thursday.

"To a great degree, a presidency shapes the public character of the nation," Romney wrote. "A president should unite us and inspire us to follow 'our better angels.' A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect. 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."

Romney writes in the editorial that he hoped Mr. Trump "would rise to the occasion" after the election, and he writes that he approved of some of Mr. Trump's early appointees. But Romney calls the departures of White House chief of staff John Kelly and Defense Secretary James Mattis part of the "deep descent" of the presidency in December.

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Although Romney is critical of Mr. Trump in the editorial, he also writes that he will follow the leadership of Senate Majority Leader Mitch McConnell, who has been aligned with Mr. Trump in the past. Romney also doesn't give any specifics on how he will oppose Mr. Trump while in Washington.

President-elect Donald Trump and Mitt Romney shake hands at the entrance of Trump National Golf Club Bedminster clubhouse in Bedminster, N.J., Saturday, Nov. 19, 2016, after meeting. Carolyn Kaster / AP

The president responded to the scathing editorial on Wednesday morning.

"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!," Mr. Trump wrote, referring to outgoing Sen. Jeff Flake, R-Arizona, a constant critic of the president.

Mr. Trump's 2020 campaign manager, Brad Parscale, tweeted that Romney "lacked the ability to save this nation ... Jealousy is a drink best served warm and Romney just proved it."

The truth is @MittRomney lacked the ability to save this nation. @realDonaldTrump has saved it. Jealously is a drink best served warm and Romney just proved it. So sad, I wish everyone had the courage @realDonaldTrump had. https://t.co/mbxoTqbSX6 — Brad Parscale (@parscale) January 2, 2019

Romney sharply criticized Mr. Trump during the 2016 election but then met with him during the transition period before Mr. Trump took office. Romney also accepted Mr. Trump's endorsement in the 2018 election.

In an interview with CNN on Wednesday, Romney reiterated that he was willing to work with the president when he agreed with him and criticize him when they disagree.

"It's important as I begin this new job to make it very clear where I stand," Romney said. However, he also expressed support for a border wall, one of Mr. Trump's biggest priorities. Mr. Trump is currently refusing to sign a government funding bill which does not provide sufficient money for the wall.

"I would vote for the border wall. I've made that part of my platform for many years," Romney said, adding that he was "with the Republicans."

Mr. Trump's approval rating has dropped the most in Utah compared to any other state, according to the Salt Lake Tribune in October. According to the newspaper, 58 percent of Utah residents disapprove of Mr. Trump and only 45 percent still back the president.

Although Mr. Trump won Utah by 18 points, Romney won the deep-red state by 48 percentage points in 2012. In the 2016 election, anti-Trump Independent candidate Evan McMullin won more than 20 percent of the vote in the state.