Eliana Johnson of National Review Online explores a chink in Florida Sen. Marco Rubio’s armor that could hurt his presidential bid.

Rubio’s shaky performance Saturday is likely to cement the impression that he lacks an intangible quality, some part of that elusive amalgam that makes a candidate “presidential.” For months, reporters following Rubio on the campaign trail have labored to put their finger on it. Many have said he’s too sunny and too polished for the current Republican moment. Yet others have said that he simply looks too young. National Review’s Rich Lowry argued in January that Rubio has an “affect problem” — that he’s smooth and charming, sure, but he’s not angry, harsh, or biting when the moment demands it. When he’s tried to be, his attempts feel strained.

One Republican operative close to the Rubio campaign puts it to me this way: Rubio, he says, has “zero swagger, zero cockiness.” Another political observer says what he lacks is stage presence, an ability to appear as if he’s “quietly dominating everybody in the room.” And it’s true, Marco Rubio will never gut a challenger with a smirk and a nod, as George W. Bush did to Al Gore, or by gently mocking his argument, as Ronald Reagan did to Jimmy Carter: “There you go again . . .”

Reagan later said he delivered the line spontaneously — because, well, “It just seemed to be the thing to say.” That’s probably not how Rubio and his disciplined, cautious team would explain any of their big campaign moments. When Rubio has attacked, as when he knocked Jeb Bush down on the debate stage, he has down so in a way that has been both obviously rehearsed and expertly delivered.