Washington Post columnist Michael Gerson posed an interesting question in the aftermath of the midterm elections: Would a Republican candidate dare challenge President Donald Trump in the 2020 primaries?

Gerson conceded a major obstacle to such a scenario, that "toppling a sitting president of your own party is a maneuver with the highest degree of difficulty."

Indeed, it seems like a political fantasy to suggest a sitting president could lose in the primaries. It's been 50 years since there was anything remotely close to such a situation. Democrat Eugene McCarthy used his opposition to the Vietnam War and his 42 percent showing in New Hampshire to expose President Lyndon Johnson's vulnerability. Johnson, who had served more than a full term due to the assassination of President John F. Kennedy in 1963, ultimately decided to not seek the nomination. 

There are key factors that point to Trump running unopposed in the 2020 primaries, with the most notable being that he has considerable support from Republican voters. According to Gallup's most recent poll, Trump has an approval rating of 91 percent amongst Republicans.

A Republican challenger may also feel justifiable pessimism to face Trump in the first primary. In 2016, Trump easily defeated a very crowded GOP field in New Hampshire, pulling in 35.23 percent of the vote, while his closest competitor, Ohio Gov. John Kasich, drew just 15.72 percent of the vote. 

However, some Republicans might see Trump as a weakened candidate. Not only is he facing an investigation by special counsel Robert Mueller but Trump also has a disapproval rating over 50 percent. A GOP candidate may want to take a chance and challenge Trump in the primaries should things not improve for the president in 2019.

There might be political will for a challenger, as well. In a poll conducted in November by the American Barometer, 72 percent of voters said Trump should face a primary opponent and that included 43 percent of Republicans. 

There are also a number of Trump critics within the GOP and they date back to when he was a candidate. Late into the primary season, and with Trump gaining in popularity, Kasich and Sen. Ted Cruz of Texas coordinated a failed attempt to deny Trump the nomination. It was an unprecedented move and it spoke to the level of discontent that some Republicans feel about a political outsider who embraced controversial and inflammatory rhetoric. Even little-known Evan McMullin, a former CIA officer, ran in 2016 as a conservative independent and in direct opposition to Trump. 

So who might throw their hat into the Republican primaries in 2020?

John Kasich

In an appearance on "The View," Kasich said he wasn't sure if he'd run as an independent but added, "all my options are on the table."

Kasich could make for an interesting candidate, whether as a Republican or as an independent. He is a two-term governor and a former congressman from Ohio, a highly coveted battleground state. Kasich is also considered a moderate conservative and might be an alternative for more center-leaning Republicans.

"I think for the first time, there is a legitimate chance for a third-party candidate,” Kasich said. 

He added, however, that while he might do well in New Hampshire he would have trouble winning in the southern states.

Jeff Flake

Arizona may be moving towards being a swing state, which makes Sen. Jeff Flake an intriguing potential challenger to Trump. After choosing to not defend his Senate seat in 2018, there have been rumblings that Flake's political career isn't over.

"I’ve not ruled it out. I’ve not ruled it in. Just, somebody needs to run on the Republican side," Flake told Politico and The Hill earlier this month about the 2020 election.

Flake, who represented Arizona alongside noted Trump critic John McCain, has helped represent the wing of the GOP that has expressed disapproval for Trump's behavior and some of his policies. During the heated nomination process for Supreme Court Justice Brett Kavanaugh, Flake appeared to affirm both his conservative and moderate leanings by voting for Kavanaugh but also asking for an FBI investigation into allegations against the then-nominee. 

"I hope somebody does [run], just to remind Republicans what it means to be conservative and what it means to be decent. We’ve got to bring that back," Flake said. "You can whip up the base for a cycle or two but it wears thin. Anger and resentment are not a governing philosophy."

Sen. Ben Sasse

Sasse could emerge as a maverick candidate. Nebraska is a very red state but Sasse has expressed disapproval of not just Trump but also his party. When pressed by CNN's Jake Tapper, Sasse didn't seem to close the door on running as an independent and also said he considers leaving the GOP.

But Sasse seemed to throw cold water on a Republican challenging Trump in 2020.

At a National Press Club Headliners luncheon, Sasse said Trump "has basically captured the majority of the Republican Party over the course of the last two and one-half years. The Republican electorate is pretty comfortable with the anti-positions."

Others

Three other names have drawn interest but all seem a bit far-fetched. 

Sen. Bob Corker of Tennessee, who is leaving the Senate in January, has been a noted Trump critic. Sen.-elect Mitt Romney of Utah, who won the Republican nomination in 2012, has not been shy about criticizing Trump and has even called him "a fraud."

Nikki Haley, who served as Trump's ambassador to the United Nations, has also been mentioned as a possible candidate. She is the former governor of South Carolina, which is expected to remain a red state in 2020.

Would A Republican Challenger Hurt Trump's Re-Election Hopes?

There may be some conflicting views as to how a Republican primary challenger would ultimately influence the general election. 

In some ways, Trump would relish such a challenge. He has eagerly sought to speak at rallies during his nearly two years as president and has jumped on the opportunity to belittle his opponents, no matter their party. 

Democrats may like the idea of a Republican candidate to challenge Trump since the last two one-term presidents faced some opposition in the primaries. In 1980, President Jimmy Carter faced Sen. Ted Kennedy of Massachusetts in the Democratic primaries and then lost to former California Gov. Ronald Reagan in the general election. In 1992, President George H.W. Bush lost to Arkansas Gov. Bill Clinton in the general election after he faced former Richard Nixon advisor Pat Buchanan in the Republican primaries. 

Sportsbook BetOnline lists the Democratic Party as the favorite to win the 2020 presidential election at -145, while the Republican Party is listed as a +125 underdog.