WASHINGTON (AP) — Mitt Romney, the 2012 GOP presidential nominee and incoming Utah senator, has quickly set himself apart from other Republicans in the new Congress with a blistering attack on President Donald Trump's leadership and character, triggering what could become a new rivalry in the GOP ranks.

Romney put to rest expectations that he would take his time getting his footing in Washington. Instead, he penned an op-ed in The Washington Post, published two days before being sworn into office, that said 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."

Trump replied by tweeting he hoped Romney wouldn't follow in the footsteps of Sen. Jeff Flake, R-Ariz., who often criticized Trump and paid the price, opting to retire rather than risk defeat in a GOP primary.

"Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful," Trump tweeted. "I won big, and he didn't. He should be happy for all Republicans. Be a TEAM player & WIN!"

By taking on Trump so early in his Senate career, Romney could be picking up where Flake and Sen. Bob Corker, R-Tenn., another retiring senator, left off.

Both Republicans were vocal GOP critics of Trump, and their retirements left some wondering whether any other Republicans would be willing to publicly criticize the president. Sen. Dick Durbin, D-Ill., noted that almost half the Senate Republicans are up for re-election in 2020 and some may feel the need to push back against Trump.

"They just saw what happened in 2018," Durbin said, referring to Republicans losing the House majority in November. "I think, once they do polling back home, not all of them but many of them will find that independence is being rewarded."

Romney has had his public run-ins with the president before and tried to prevent him from winning the GOP nomination in 2016. In one speech, he said there was plenty of evidence that Trump was "a con man, a fake." In that same speech, he said, "Dishonesty is Donald Trump's hallmark."

But after the presidential election, Romney eased off the criticism and interviewed to become Trump's secretary of state. Trump instead opted for former Exxon Mobil CEO Rex Tillerson, who lasted about 14 months before Trump fired him.

During his Senate campaign, Romney insisted that he would agree with Trump on some issues and not be shy about disagreeing with him on others. He appears to have more room with GOP voters in Utah to take on the president. Most voters in the state — 64 percent — would like to see the senator confront the president, according to data from AP VoteCast, a survey of midterm voters.

Romney's criticism comes during a particularly tumultuous time in the Trump presidency. The stock market tumbled in December and closed out the year with its worst showing in a decade. Trade disputes have slowed global growth. Defense Secretary Jim Mattis resigned over differences concerning Trump's policy in Syria. Disagreement over funding for a border wall led to a partial government shutdown.

Special counsel Robert Mueller is continuing his investigation into Russia meddling in the 2016 election and possible contacts with the Trump campaign.

Romney credited Trump in the op-ed for cutting corporate taxes, stripping out what he described as excessive regulation and appointing conservative judges. But he said policies and appointments are only part of being a president. He said a president must also demonstrate honesty and integrity and elevate the national discourse.

"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.

Trump said during a Cabinet meeting Wednesday that he was surprised by Romney's comments. "People are very upset with what he did," Trump said. He also referenced Romney's 2012 election loss to President Barack Obama.

"If he fought the way he fights me, I'm telling you, he would have won the election," Trump said.

In a conference call with reporters, GOP Sen. Rand Paul of Kentucky noted that the 2020 election hopes of many Republicans in the Senate and House will be tied to Trump. Paul said he disagrees with the president on some issues but still treats him with respect.

"I don't think the president deserves a new senator coming in attacking his character," Paul said.

Ronna McDaniel, chairwoman of the Republican National Committee and Romney's niece, retweeted Trump's remarks about Romney and added that the president is constantly "attacked and obstructed" by the media and Democrats.

"For an incoming Republican freshman senator to attack" Trump @realdonaldtrump as their first act feeds into what the Democrats and media want and is disappointing and unproductive," McDaniel tweeted.

Romney's rebuke of Trump also drew a cutting reply from Brad Parscale, Trump's campaign manager. In a tweet Tuesday night, Parscale said Romney "lacked the ability to save this nation" and contended that Trump "has saved it."

"Jealously is a drink best served warm and Romney just proved it," Parscale wrote, adding he wished "everyone had the courage" Trump had.

But conservative commentator Bill Kristol, a frequent Trump critic, tweeted that Romney's op-ed confirmed that "Trump's dominance over the GOP, pretty complete until now, can no longer be taken for granted."

"For now at least Mitt Romney has become the leader of the Republican Resistance to Trump," Kristol tweeted.

Romney will be sworn in as a senator on Thursday.

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