Presumably Biden runs for the Democrats, but Harris, Pritzker, Whitmer, and Newsom are all waiting as warm backups if Biden decides he doesn’t want to run. Pritzker probably gets the nomination from that group.
Trump likely tries to run for Republicans. DeSantis is probably in pole position for the nomination, but Haley has a real chance too. Hawley, Rubio, and Scott presumably run too.