Romney, who ran against former President Barrack Obama in 2012, made the comments in an essay published in the Washington Post on Tuesday evening.
“The appointment of senior persons of lesser experience, the abandonment of allies who fight beside us, and the president’s thoughtless claim that America has long been a ‘sucker’ in world affairs all defined his presidency down,” he wrote.
He added that “Trump’s words and actions have caused dismay around the world.”
Trump fired back Wednesday morning, calling Romney a “flake” and saying he should be a team player and “be happy for all Republicans.”
WATCH: N.Y. Gov. Andrew Cuomo criticizes President Trump in inauguration speech on Ellis Island
*More to come.