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Sen.-elect Mitt Romney wasted no time in the new year branding himself as a Senate maverick and a foe of President Donald Trump, saying in a blistering New Year's Day essay that after nearly two years in the White House "the president has not risen to the mantle of the office."

Romney, just two days from being sworn in as the junior Republican senator from Utah, wrote in the Washington Post that Trump's most glaring failure has been his inability to unite a "nation so divided, resentful and angry."

"It is well known that Donald Trump was not my choice for the Republican presidential nomination," Romney said. "After he became the nominee, I hoped his campaign would refrain from resentment and name-calling. It did not."

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! — Donald J. Trump (@realDonaldTrump) January 2, 2019

Trump responded on Wednesday morning asking if Romney were "a Flake," referring to Jeff Flake, the retiring GOP senator from Arizona who has been a sharp critic of the president.

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Romney and Trump have had a publicly volatile relationship, going back to the 2016 presidential campaign when Romney re-emerged in national politics to call Trump "a phony, a fraud," who was "playing the members of the American public for suckers."

Then-President-elect Donald Trump and Mitt Romney dine at Jean Georges restaurant in New York on Nov. 29, 2016, as Trump weighed who to pick as his first secretary of state. Drew Angerer / Getty Images file

Then-candidate Trump punched back, saying that Romney had begged for Trump's endorsement during his own 2012 presidential run against former President Barack Obama. "I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees," Trump said at the time.

In a widely perceived effort at revenge through public embarrassment after the November 2016 election, Trump made a spectacle of dining with Romney as he weighed his choice for secretary of state, only to give the job to Rex Tillerson.

And in May of last year, Romney told NBC News that he does not consider the president a role model for his grandchildren.

In the op-ed published online Tuesday, Romney credited Trump with having some positive policy and hiring agendas, but said "policies and appointments are only a part of a presidency."

Romney's salvo places him in a unique position among GOP members of the Senate. With Jeff Flake of Arizona and Bob Corker of Tennessee retiring, the remaining Republicans in the chamber have largely shied away from criticizing Trump.

Romney telegraphed that he intends to be more outspoken. While saying he would not address each one of Trump's more controversial comments and Tweets, he pledged to "speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."

The incoming Utah senator promised to work with Senate Majority Leader Mitch McConnell, R-Ky., and other senators.

As it regards Trump, Romney said, "I will act as I would with any president, in or out of my party: I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not."

Brad Parscale, Tump's 2020 campaign manager, hit back at Romney on Twitter Tuesday night.

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

"Jealously is a drink best served warm," Parscale said.