The GOP’s cranky side seems to bother him. He argues that the Republican Party needs to offer up clear alternatives to liberal policy, not just say no, and brighten its tone along the way. Take immigration. “Where Republicans have failed: We should be the pro-legal immigration party, not the anti- illegal immigration party,” he says. If he wins, Mr. Rubio will be the most prominent elected Hispanic official in the U.S. from either party…

Coming from a Bush, the joke brings home another truth. The GOP hasn’t been blessed with good communicators the last two decades. Mr. Rubio is articulate, highly disciplined and gaffe-free. With audiences he projects a warmth harder to feel in person, where he can come off cool, somewhat aloof, unless you get him talking about his love for football. At 39, the Gen-Xer represents a new generation of Republican leaders.

Democrats who knew him in his days as speaker of the Florida House of Representatives in 2007-09 shake their heads at the transformation. At first, the local press didn’t take his candidacy seriously. The campaign went around them and the party establishment. The Web and the tea parties helped. The national conservative press did as well by bringing him the sort of attention he was denied in Florida.