"I do believe if the president is running for re-election, if he continues on the path that he's on, that that's going to leave a huge swath of voters looking for something else," Jeff Flake, one of the few elected Republicans to publicly criticise Trump, said in an interview on ABC's "This Week."
"If he's the Republican nominee again, we're likely to see an independent candidate" in the November 2020 presidential election, said Flake, who announced this fall that was stepping down from his Senate seat in late 2018.
"He's probably inviting a Republican challenge as well" for the 2020 primaries to tap a nominee for the Republican party, he warned.
Sitting presidents are typically considered shoo-ins as the nominee for their party at the end of the first term, although an intra-party challenge is not unheard of.
Democratic President Jimmy Carter faced off against Senator Ted Kennedy in 1980, and four years before that, Republican President Gerald Ford clashed with Ronald Reagan.
Flake is among the few Republican lawmakers who have publicly criticised the president, both on a personal level and for his policies which the senator deems extreme.
"You look at the audiences cheering for Republicans... you look out there and say, those are the spasms of a dying party," he said in the interview.
"By and large, we're appealing to older white men. And there are just a limited number of them. And anger and resentment are not a governing philosophy."
Flake was circumspect when asked if he was considering a run in 2020.
"I don't rule anything out but it's not in my plans," he said.
Congressman Charlie Dent, a leading Republican moderate in the House of Representatives who is also stepping down next year, on Sunday denounced what he characterised as the party's blind loyalty to Trump.
"The issue is loyalty to the man. To the president. And for some, loyalty is not enough. You have to be angry and aggrieved," he said on ABC.
"I have said to folks, if I set myself on fire for them, they would complain that the temperature of the flame is not hot enough."
Dent said that even though he expects Republicans to maintain their legislative majority in mid-term elections in November 2018, the party can still expect to lose seats.
His advice to colleagues: "Be prepared for the worst because this could be a really tough year."
