He is this generation's Dan Quayle.
Follow me beyond the orange squiggle of justice, and I will elaborate.
Paul Ryan is in serious danger of following the same career path as Quayle. Both men were picked as running mates by late-middle-aged establishment figures. The selections followed the same logic - both Bush I and Romney were facing younger, more energetic candidates. Neither had established full credibility with the conservative base of the party.
The theory was that adding a rising star from the right-wing of the party as VP would be a signal to the base that the candidate was not just paying lip-service to their ideals. It also gives the right a consolation prize - their guy would be setup as the heir apparent.
While the same logic could have applied to the Palin selection, there was a big difference. Ryan and Quayle had extensive legislative experience, and had been building towards prominence piece by piece. Palin was never considered an expert on anything. Her reputation was not destroyed by her candidacy - in fact, it was enhanced in some circles.
Quayle ended up being derailed by being thrust into the spotlight before he was ready. He likely would have been better served by raising his profile and national stature in the Senate before going under the microscope of a Presidential campaign. An article published in the New York Times, August 29, 1988 shows that he wasn't regarded as a light-weight.
Paul Ryan entered the campaign with a reputation in the conservative and mainstream media as an intellectual. That reputation is in serious jeopardy - his brand is closer to Eddie Haskell at this point than William Buckley. If Romney loses, that will be the image of Ryan going forward.
UPDATE after some comments: We will know more after the debates. Dan Quayle was ruined by Lloyd Bensten in 1988. He did better against Gore - if not on the facts, at least he looked less overwhelmed than he did in '88. But the damage was done. If Biden bulldozes Ryan the way that LB bulldozed DQ, young Paul will rue the day he answered Mitt's email.