According to The Hill:

Will President Trump face a challenge from within his own party in 2020?

Arizona Sen. Jeff Flake (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. 

The five Republican challengers include Flake, Sen. Ben Sasse (R-NEB), New Mexico Governor Susana Martinez (R), Ohio Governor John Kasich (R), and Sen. Ted Cruz (R-Texas).

The names may change but according to The Hill, there will be plenty of challengers.