In a tweet, the president called on Romney to "focus on Border Security and so many other things where he can be helpful," while questioning whether he be "a Flake," referencing frequent Trump Sen. Jeff Flake (R-Ariz.), who did not run for reelection last year.
"Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not," Trump wrote Wednesday morning.
"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!" he added.
The president's tweet came hours after Romney targeted him directly in a Washington Post op-ed, accusing him of a glaring "shortfall" in terms of character and leadership abilities.
"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 wrote Tuesday.
"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 continued.
Elected in November to succeed retired Sen. Orrin Hatch (R-Utah), Romney is due to be sworn in as Utah's junior senator on Thursday.