I do fully acknowledge the question may be ridiculous since it could very well be someone most of us have never heard of (IE- a Virginia state legislator who goes on to win the next gubernatorial election.) Trying to think of the people who have a more than ten percent chance, I'm wondering if it's Mike Pence or Elizabeth Warren.
Warren is probably one of the two likeliest Democratic nominees, and the likeliest to run for reelection. My sense is that she's a relatively weak general election candidate, but it doesn't mean there's no chance she'd be a reelected President. You could imagine her running in 2024 as Biden's VP or after a Biden or Sanders loss.
Pence would be a top-tier candidate if Trump is reelected or isn't. There's also a non-trivial chance that he would be running as an incumbent President.
I'll go with Harris. She has perhaps the best chance chance of succeeding a one-term Dem, a good chance of beating a Republican in a scenario where Republicans win in 2020, and a decent shot at being the incumbent.
That's a good argument.
She hasn't really taken off in the current environment, though.
I think, no matter who wins in 2020, Ron DeSantis has a better chance than anyone to be elected in 2024.
If Trump is reelected, DeSantis would have to defeat other top-tier candidates and win a third term for his party (a feat that's only been accomplished once since Ike got the White House.)
If Trump loses, there may be an open election due to some older Democratic frontrunners, but this would mean DeSantis would need to win a primary that could include other Floridians (Rubio, Scott) in addition to winning a General Election after the other party has been in the White House for only one term.
