Mitt Romney was so anxious to launch his Washington career with an anti-Trump hand grenade that he couldn't even wait until he was sworn in.

The newly elected senator from Utah is taking plenty of heat for attacking the president in a Washington Post op-ed, but he produced the desired explosion in the New Year's Day lull.

With such critics as Bob Corker and Jeff Flake leaving the Senate, Romney immediately established himself as the leader of the GOP's anti-Trump resistance.

The mainstream media, naturally, is delighting in the spectacle of the Republicans' 2012 standard-bearer denouncing the nominee who actually won the White House. Many journalists and pundits trashed Romney six years ago, portraying him as an awkward, heartless, out-of-touch plutocrat. But now they suddenly recognize his virtues as a man of seasoned judgment.

It doesn't take a deep dive to recognize that the onetime Massachusetts governor has been all over the map when it comes to Trump.

Romney happily accepted Trump's support when he was the 2012 nominee. But when Trump was the standard-bearer, Mitt called a press conference to denounce him as a "fraud" and "phony" whose trademark was "dishonesty ... bullying, the greed, the showing off, the misogyny, the absurd third-grade theatrics."

And yet when Trump won, Romney was happy to have dinner with the president-elect and audition for the secretary of State job (which of course was a mirage). And he was just as happy to accept Trump's endorsement for his Senate run a year ago.

But that was then, this is now.

In the Post column, Romney says Trump's "conduct over the past two years, particularly his actions last month, is evidence that the president has not risen to the mantle of the office."

"A president should unite and inspire us," leading with honesty and integrity, “and it is in this province where the incumbent’s shortfall has been most glaring.”

And then he pulled the pin and hurled the grenade: "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."

Of course, the press will ask him to comment on every tweet, as Romney well knows. That piece wasn't a series of jabs, it was a roundhouse right.

The president was unusually restrained on Twitter, saying: "Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not." He added that he had won big and Romney didn't and that the new senator should "Be a TEAM player & WIN!"

There's been some chatter about whether Romney is positioning himself for a primary challenge. I doubt it since Trump's hold on Republican voters is such that Romney would almost definitely fail. By the same token, he takes no risk now because he's more popular than Trump in Utah.

Perhaps the most delicious part for the press was the comment by RNC Chairwoman Ronna McDaniel that it was "disappointing and unproductive" for an incoming Republican senator to attack POTUS as his first act.

That would be the woman who once used her full name, Ronna Romney McDaniel ... talking about her uncle.

Mitt has chosen sides, and the press is siding with him. What's unclear is how it affects his clout as one of 100 senators.