Senator John Kasich - 55.6%
Senator Benjamin Sasse- 24.9%*
Former Speaker Newt Gingrich - 19.5%**
*Dropped out May 28, two days after losing Oregon with under 40%
**Dropped out March 24, after losing Utah
Newt Gingrich will be 76 at the start of the 2020 primaries; I doubt that he will try to run again.
I have him there to essentially meld Gingrich 2012/Trump 2016 primary voters. The only other candidates who might be able to pull it off are Bobby Jindal and Rick Scott, who are both pretty unpopular. Sessions doesn't seem like a national candidate to me. I don't think he'd try it.
Maybe if Andre Bauer gets elected in 2018, he could do it. Otherwise, Gingrich is the main Deep Southern/Eastern Southern Republican who endorsed Trump with charisma.