Sen. Marco Rubio was so over the Senate that he missed more than half of the chamber’s votes in late 2015 and repeatedly mocked his colleagues for being useless. Now, the Florida Republican is giving serious thought to revoking his retirement and running to rejoin his fellow senators for another term. Kyle Kondik of the University of Virginia’s Center for Politics said Mr. Rubio would have his work cut out for him in the primary and general election if he runs. “Rubio’s star has dimmed after his presidential race, and he could potentially face problems in both a primary...