Mr. Rubio’s most persuasive pitch to Republican Party insiders may well be that he is more popular than other, ideologically similar candidates. Some of those candidates, like Mr. Ryan, can probably offer a richer intellectual defense of conservatism, or can claim to have been better vetted. Several others, like Gov. Bobby Jindal of Louisiana, have more executive experience. Mr. Rubio’s relatively favorable public image represents his comparative advantage. (There are also the facts that Mr. Rubio is Hispanic and is from Florida, but these advantages boil down to electability as well: the possibility that he might help Republicans make gains with Latinos, and that he could give them a lift in an especially important swing state.)

What makes matters tricky for Mr. Rubio is that, at the same time he is hoping to persuade Republican party insiders that he deserves their support, he will also need to maintain a reasonably good image with the broader electorate lest his electability argument be undermined. This may lead to some strange positions, such as when Mr. Rubio recently critiqued President Obama’s immigration proposal despite its many similarities to his own.

When the wider electorate learns that Mr. Rubio’s positions are in fact hard to differentiate from those of other conservative Republicans, will his favorability ratings revert to being mediocre, as Mr. Ryan’s now are?