It’s fair to say that there is skepticism that Trump will run, or be a viable general election candidate, in 2020, prompting a healthier-than-usual amount of ground testing by multiple potential 2020 candidates, including VP Mike Pence, Gov Kasich, and Senators Tom Cotton (Arkansas) and Ben Sasse (Nebraska).  This article also mentions UN Ambassador Nikki Haley, who would be a strong candidate.

http://www.msn.com/en-us/news/politics/republicans-make-moves-for-2020-as-trump-doubts-grow/ar-AAptfJ0?li=BBmkt5R&ocid=spartandhp