As President Trump stumbled through a rocky month of staffing upheaval, controversy, and public feuds, hypothetical talk of replacing him as the Republican nominee for president in 2020 has grown louder despite the improbability of the prospect.

No sitting president in the modern era has been replaced during his re-election race by a challenger from within his own party. While some have faced primary challenges — Ronald Reagan mounted a strong challenge against Gerald Ford in 1976, Ted Kennedy challenged Jimmy Carter in 1980, and Pat Buchanan ran against George H.W. Bush in 1992 — none have succumbed to those intra-party challengers.

But several high-profile Republicans speculated openly this week someone other than Trump could represent Republicans on the 2020 ticket.

Sen. Jeff Flake, R-Ariz., said on Wednesday Trump "is inviting" a 2020 primary challenge by "drilling down on the base rather than trying to expand the base."

Rep. Will Hurd, R-Texas, refused on Thursday to say definitively he would support Trump as the Republican nominee in 2020.

And reports of a so-called " unity ticket" involving Republican Gov. John Kasich and Democratric Gov. John Hickenlooper surfaced Friday, further stoking speculation Republicans are considering their options if the president remains weak heading into 2020.

"Since 1900, 20 presidents have sought re-election and of those, 15 have won and five have lost. So basically, he's the favorite right now even with the numbers the way they are," said Ford O'Connell, a Republican strategist. "The only question is whether he chooses not to run again and that's it."

Trump's approval ratings have fallen to new lows amid controversy surrounding his response to racial violence in Charlottesville, Va. earlier this month and the ongoing stagnation of his legislative agenda. His escalating attacks against members of his own party — including Senate Majority Leader Mitch McConnell, House Speaker Paul Ryan, and Sen. Bob Corker — have weakened his already fragile bonds with GOP lawmakers.

Grant Reeher, a political science professor at Syracuse University, said a primary challenge to Trump would be futile unless Republicans could "have an agreed-upon person that's going to represent that challenge."

"They'd have to unite behind someone, and that was something they were not able to do in 2016," Reeher said, referring to last year's Republican presidential primary, in which more than a dozen GOP contenders attempted to prevent Trump from winning the nomination.

"You could argue that Ted Cruz is the highest-profile leader of the most conservative wing of the Republican Party, but there's not unanimity about that view," Reeher added.

Although speculation about a potential Republican replacement for Trump in 2020 became more visible this week, Trump has always faced resistance from Democrats, from many segments of the media, and even from within his own party.

A GOP strategist who declined to be named argued the proliferation of talk about a 2020 primary challenger for Trump is a tempting story for the media to cover because it undermines Trump's legitimacy.

"The mainstream media loves to call into question President Trump's viability. Viability now, viability in 2020," the strategist said. "This is their agenda. History does not support any suggestion that a sitting president can be defeated for his own nomination."

But Jack Rakove, an American history professor at Stanford University, said Trump is such an anomalous figure that the laws of history may not apply to him.

"To compare a Trump presidency to any other presidency just doesn't make any sense. It's just so distinctive, unique in every imaginable respect," Rakove said.

"There's no precedent for this situation, there's no precedent for a guy who was really outside the apparatus of the party, and then coercing the party to follow his lead," Rakove added.

While people "might be talking about it widely," Rakove said no Republican is likely to begin "playing for the presidency" until after the midterm elections.

"A lot of challenges would be predicated on what happens in 2018, if the Republicans really take a bath," he said.

For now, however, Trump still holds a massive advantage over his potential challengers.

Public Policy Polling, a left-leaning firm, found this week 57 percent of Republicans still want Trump to be the Republican nominee in 2020.

The PPP poll found Trump held a 28-point lead over Vice President Mike Pence, a 40-point lead over Cruz, and a 47-point lead over Kasich in hypothetical primary match-ups.

Tony Fabrizio, Trump's campaign pollster, shared a poll on Wednesday that also found Trump holding major leads over Cruz and Kasich in a 2020 primary. Sen. Ben Sasse, a prominent GOP critic of Trump, commanded just 1 percent of the vote in the hypothetical primary contest.