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Donald Trump declared himself “the most popular president in the history of the Republican Party” on Wednesday. Yet his allies fear a primary challenge from a high-profile Republican could doom his re-election.

The concern was outlined in a private email shared among Republican National Committee members hours after the GOP’s last failed presidential nominee, Mitt Romney, lashed out at Trump’s character and global leadership in an opinion article published in The Washington Post. Romney is set to be sworn in as Utah’s junior senator on Thursday.

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His scathing message was widely interpreted as a sign of encouragement for Republicans including outgoing Ohio Gov. John Kasich and retiring Arizona Sen. Jeff Flake to take on Trump. While a successful primary challenge is highly unlikely given Trump’s grip on the GOP base, some fear it would consume energy and resources badly needed by Trump, who is facing record-low approval ratings and signs of revolt among suburban voters.

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Win or lose, any primary challenge would almost certainly hurt Trump’s re-election, warned RNC member Jevon Williams of the Virgin Islands.

“Messrs. Romney, Flake, and Kasich will continue chasing their fantasy of being president, even if that means destroying our party and denying President Trump re-election,” Williams wrote to fellow RNC members in a message obtained by The Associated Press. “Look, the political history is clear. No Republican president opposed for re-nomination has ever won re-election.”

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Kasich was clearly encouraged by Romney’s criticism of the president.

“Welcome to the fray, @MittRomney,” Kasich wrote in a Twitter post Wednesday sharing Romney’s article.

Kasich adviser John Weaver said it’s been “awful lonely” for Kasich in recent years as one of the GOP’s most vocal Trump critics.

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“It’s not so lonely now,” Weaver said.

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Kasich, who leaves office later this month, is taking steps to strengthen his organization ahead of a possible run by adding paid staff and volunteers to his political and finance teams.

His advisers say he has yet to decide whether to challenge Trump — either as a Republican or as an independent — although Kasich was quick to visit New Hampshire, the base of his last presidential run, after the November midterms.

The debate among Republicans is how — and whether — to protect Trump. Williams called on his RNC colleagues to change party rules to make it harder for a viable primary challenge to take shape.

Currently, any candidate who wins a plurality of delegates from five states can be nominated from the national convention floor. That number, which was higher in past years, was lowered at the 2016 convention in response to concerns from grassroots activists. Now some of those activists, loyal to Trump, fear the potential consequences.

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Incumbents in either party bring universal name recognition and massive institutional support to their re-election campaigns, and an insurgent effort to deny Trump the nomination would almost certainly fail. But there is still ample room for a potential spoiler to enter the race to try to deny Trump the support needed to win the general election.

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Primary challengers in 1980 and 1992 helped weaken then-Presidents Jimmy Carter and George H.W. Bush, who were ultimately defeated in November.

Still, little has come of recent discussions in South Carolina and New Hampshire, where local officials hoped to take steps to block potential Trump challengers. RNC officials now say the national GOP’s rules cannot be changed before the 2020 national convention.

Former RNC member Saul Anuzis of Michigan said Trump’s team missed an opportunity.

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“Nobody wants a primary. It’s not healthy for us,” Anuzis said. “You’ll have the Democrats attacking Trump and some Republicans attacking Trump. How does that help us?”

Trump, whose party suffered deep losses in the November midterm elections, shrugged off the prospect of a serious primary challenge.

“They say I am the most popular president in the history of the Republican Party,” he told reporters.

Gallup found late last month that just 39 percent of Americans approved of Trump’s job performance, a mark lower than any president in either party at this point in his presidency since at least 1954. That said, 89 percent of Republicans approved of Trump’s job performance.

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Trump’s behavior could shift sharply in the second half of his first term as he prepares for the 2020 contest.

The president is facing pressure from the special counsel in the Russia probe and an impending onslaught of Democratic investigations. That could push Trump to cater exclusively to the base of voters he is concerned about losing, according to a Republican close to the White House who has consulted on the early re-election efforts.

That instinct would echo the president’s double-down, scorched-earth response to the crises that hit his 2016 campaign, including the “Access Hollywood” tape about forcing himself on women, and could make it harder to woo the independent voters or disaffected Democrats he may well need.

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The president is eager to unleash his re-election machinery and has pushed to collect pledges of loyalty from across the GOP to quell any hint of an insurrection, according to a campaign official and a Republican familiar with the inner workings of the campaign. Neither was authorized to speak publicly and both requested anonymity.

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The Trump team has discussed the possibility of a challenge from someone such as Kasich or Flake and now, one adviser said, Romney would enter the conversation.

Most in Trump’s orbit were not worried about Romney, but others worried that the new article, which was published to great buzz, was merely the opening salvo in coordinated effort by some Republicans to deny the president the party’s nomination.

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Publicly, RNC officials were united behind the president.

“There is no concern or expectation at the RNC of a primary challenge for President Trump at all,” said RNC member Bill Palatucci of New Jersey, a state where Republicans suffered painful losses in November. “There may be disagreements, but you express those disagreements in private. As difficult as a midterm cycle that we had, everybody still wants a Republican president and not Elizabeth Warren or Bernie Sanders.”

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Romney himself insisted that his article was not intended to signal any interest in a third Republican presidential bid, but he declined to endorse Trump’s re-election.

“I haven’t decided who I’m going to endorse in 2020,” Romney said on CNN. He continued: “I’m not running again. We’ll see if someone else does in a Republican primary or not. Time will tell.”