Will President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE face a challenge from within his own party in 2020?

Arizona Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE (R) seems to think so. On Sunday, Flake told ABC’s “This Week” that if the president did not change his approach, he “is going to leave a huge swath of voters looking for someone else.”

Flake, a frequent Trump critic, added that the president is “probably inviting a Republican challenge.”

Any challenger to Trump would face enormous hurdles.

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The president remains popular among Republican voters, even as he endures historically low approval numbers at large. In most polls, around 80 percent of GOP voters give Trump the thumbs-up.

Still, Trump would become much more vulnerable if Republicans suffered major losses in the midterm elections set for November 2018.

And it’s easy to see why some of his most strident GOP critics might think it’s worthwhile to run against him, even if victory is a long-odds bet.

But who is most likely to do it?

Sen. Jeff Flake (R-Ariz.)

In his “This Week” interview, Flake conspicuously left the door open to a White House run.

“I don’t rule anything out,” he said, even as he acknowledged a 2020 run was not currently in his plans.

Flake is one of the foremost Trump foes. He launched a broadside against the president from the Senate floor in October as he announced he would not run for reelection next year.

“Reckless, outrageous and undignified behavior has become excused and countenanced as ‘telling it like it is,’ when it is actually just reckless, outrageous and undignified,” Flake said. “And when such behavior emanates from the top of our government, it is something else: It is dangerous to a democracy.”

Flake would face serious challenges if he were to try to defeat Trump.

A Morning Consult poll in October put Flake’s approval rating in Arizona at just 30 percent, and only 37 percent among the state’s Republican voters — evidence that the appeal of an anti-Trump Republican is highly questionable.

But as the author of a recent book outlining an alternative path for the GOP, Flake might think it worth mounting a bid nonetheless.

He has little to lose from such a quest.

Sen. Ben Sasse Benjamin (Ben) Eric SasseMcEnany says Trump will accept result of 'free and fair election' McConnell pushes back on Trump: 'There will be an orderly transition' Trump says he'll sign order aimed at protecting premature babies in appeal to religious voters MORE (R-Neb.)

Sasse has taken the fight to Trump on his own territory: Twitter.

Responding to a June tweet from the president that alleged that he had seen TV anchor Mika Brzezinski bleeding as a result of cosmetic surgery, Sasse tweeted, “Please just stop. This isn’t normal and it’s beneath the dignity of your office.”

In October, Sasse released a statement wondering whether Trump was “recanting” his oath to protect the Constitution, in particular the First Amendment, after the president said it was “disgusting the press is able to write whatever they want.”

Some Trump critics view Sasse with skepticism, saying he has been all talk and little action in terms of dissent. They note he has rarely bucked the administration’s line when it comes to actual votes, for example.

In a July profile on Slate, Sasse was described as standing out both because of his willingness to criticize Trump and because of “doing nothing of substance as the things he says he believes in are thrown in a garbage can by his own party.”

A challenge to Trump would be the ultimate rebuttal to that kind of criticism.

New Mexico Gov. Susana Martinez (R)

Martinez and Trump have a checkered history.

In May 2016 then-candidate Trump appeared at a rally in Martinez’s state, only to criticize her. “She’s got to do a better job, OK?” Trump told a crowd in Albuquerque.

At that point, Martinez had already been critical of Trump’s rhetoric on immigration.

Later in the campaign, after the infamous “Access Hollywood” tape emerged in which Trump boasted of grabbing women by the genitals, the New Mexico governor said she could not support him. She also accused him of “a pattern of disturbing conduct and offensive rhetoric.”

Since Trump was inaugurated, Martinez has not been nearly so critical as some others, such as Flake or Sasse.

After meeting with Trump and Vice President Pence in February, Martinez praised Trump’s capacity to improve national security and strengthen the military.

Still, Martinez could be an appealing figure for anti-Trump Republicans for many reasons. As a Hispanic woman, she is the embodiment of a more diverse GOP that many in the party would like to see.

Ohio Gov. John Kasich (R)

Kasich, one of the many Republicans Trump vanquished in the 2016 primary, has been less coy than most about the possibility of making another run for the White House.

An October New York magazine profile insisted he was “plotting his path” for another campaign.

“I have a right to define what it means to be a conservative and what it means to be a Republican. I think my definition is a lot better than what the other people are doing,” Kasich told the magazine.

As the governor of a key battleground state, and one whose approval ratings were still very healthy this year, Kasich could have a decent case to make.

One big problem, however: The Republican electorate was resistant to his appeal in 2016. His home state’s primary was the only contest he won. It is not clear why a 2020 run would work out differently for the 65-year-old governor.

Sen. Ted Cruz Rafael (Ted) Edward CruzThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Cruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Texas)

The conventional wisdom is that a challenger against Trump is likely to come from a more centrist position. But conventional wisdom has usually been wrong when it comes to this president.

A challenge from the right might have more of a chance of success, given the weak position of centrists within the GOP right now.

It is easy to see the contours of the argument a hard-right figure such as Cruz could make against Trump: that the president’s abrasive personality and lack of political experience are ultimately self-defeating, both for him and the party.

Cruz has given no real indication that he might be pondering another bid. He has been broadly supportive of Trump this year after a bitterly divisive battle for the nomination in 2016.

But no one in the political world doubts the intensity of Cruz’s ambitions. If Trump were to be wounded by bad midterm results or by damning findings in Robert Mueller’s Russia probe — or both — it’s quite plausible to imagine Cruz or another conservative challenger emerging.

The Memo is a reported column by Niall Stanage, primarily focused on Donald Trump’s presidency.