NEW YORK - 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.

GOP Confronts Anxiety About Trump Primary Challenge

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.

GOP Confronts Anxiety About Trump Primary Challenge

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

GOP Confronts Anxiety About Trump Primary Challenge

"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.''

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

"Welcome to the fray, (at)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.

"It's not so lonely now,'' Weaver said.

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.

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.

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.

"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.

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