Trump on Wednesday responded via Twitter, calling for the incoming senator from Utah to "Be a TEAM player & WIN!"
In Romney's op-ed, published in The Washington Post, saying Trump has not "risen to the mantle" of the presidency and accused him of causing distress around the globe.
"His conduct over the past two years, particularly his actions this month, is evidence that the president has not risen to the mantle of the office," the soon-to-be junior senator wrote. "A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect."
He went on, "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."
“I do not intend to comment on every tweet or fault. But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions,” he added.
Romney did praise a number of Trump appointments who have since left -- naming former U.S. Attorney General Jeff Sessions, former U.S. Ambassador the U.N. Nikki Haley and ex-Secretary of State Rex Tillerson as "encouraging" choices -- and he commended some of the president's policies.
"He was right to align U.S. corporate taxes with those of global competitors, to strip out excessive regulations, to crack down on China’s unfair trade practices, to reform criminal justice and to appoint conservative judges," he said. "These are policies mainstream Republicans have promoted for years. But policies and appointments are only a part of a presidency."
Trump revisited a regular attack line on Romney in response to the op-ed, mentioning his 2012 presidential loss and questioning whether Romney would follow in the footsteps of outgoing Sen. Jeff Flake, a critic of Trump's.
"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!" Trump wrote on Twitter.
Romney straddled a line of support and disdain for Trump throughout his recent campaign for senator, and Trump endorsed Romney's campaign.
"President Trump was not the person I wanted to become the nominee of our party, but he's president now. The policies he's promoted have been pretty effective. And I support a lot of those policies. When there's a place where I disagree, I point that out," he said in October.
Ronna McDaniel, the chairwoman of the Republican National Committee and Romney's niece, offered a cryptic tweet in the wake of the column's publication, writing, "As the new Congress convenes, I hope all Rs come together to work to further @realDonaldTrump’s winning agenda and recognize his leadership has made our country safer and stronger."