Rand Paul, Marco Rubio, and Scott Walker are the three candidates in this cycle who are most likely to run in 2020 should Jeb Bush lose the general election.
If Bush wins the general election, by 2024, Paul will likely be the only one still in office - Walker would have great difficulty winning a third term and Rubio is not running for re-election to the U.S. Senate, though there is speculation he will run for Governor of Florida in 2018.
If he wants to be president, running for Governor in '18 will make no sense. Be governor for 2 years and campaign for president during the 2nd? It makes more sense to take on Bill Nelson.
Unless there's a Republican incumbent in 2020.
