Washington (CNN) Sen.-elect Mitt Romney said Tuesday in an op-ed published in The Washington Post that President Donald Trump's behavior since taking office "is evidence that the President has not risen to the mantle of the office."

"It is well known that Donald Trump was not my choice for the Republican presidential nomination. After he became the nominee, I hoped his campaign would refrain from resentment and name-calling. It did not," Romney, the 2012 Republican presidential nominee, wrote in the opinion piece, adding that several of Trump's Cabinet appointments had been "encouraging."

"But, on balance, 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," he wrote.

Romney has been a vocal critic of the President during Trump's campaign and first two years in office, although the two had appeared to mend fences after Trump was elected. They were photographed dining together as Trump mulled making Romney his secretary of state, and the President endorsed Romney's Senate bid last year.

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