It starts with the obvious: Mr. Rubio is a bright and articulate politician with the kind of broadly conservative credentials required in the Republican Party circa 2016. And his ability to break out in fluent Spanish in a news conference or a Telemundo interview gives him a chance to reach a Hispanic audience that keeps slipping away from the GOP.

Still, those attributes aren’t sufficient. For Mr. Rubio, success also depends on the magic of political timing—that is, the chance that he has arrived offering precisely what the market happens to be demanding.

On that front, his case rests heavily on two predicates. The first is that Republicans, sufficiently disillusioned with the political establishment, are ready to break tradition by trying somebody who is newer and younger—and who hasn’t waited his turn.