In all likelihood, any gains from the Ryan pick—in terms of mobilizing the conservative base—are offset (or even outweighed) by the fact that Romney has introduced a huge a new danger to his campaign. If Obama’s Medicare attacks are successful in peeling off seniors from Romney’s coalition, the path to 50+1 percent of the vote is much more difficult—if not impossible—for the former Massachusetts governor.

Indeed, as Sean Trende (who is far from easy toward Obama) points out at Real Clear Politics, this new vulnerability introduces the potential for something that was quite unlikely before—an Obama landslide. He explains:

"I’ve always thought that Obama wouldn’t be able to win more than a two-to-three-point re-election victory, mainly because a president almost never wins the votes of people who disapprove of the job that he is doing, and Obama’s approval rating is unlikely to be much above 50 percent on Election Day. But, while I don’t think it’s guaranteed, this really does give Democrats an opportunity to make Romney so radioactive that people who don’t like the president nevertheless vote for him. If the white working class revolts at the prospect of the Ryan plan, Obama really could match, or even exceed, his 2008 showing."

There’s always a tendency to assume some strategic calculation on the part of politicians—or find some reason for why a given choice isn’t a bad one—but in this case, the simplest explanation is probably the correct one. Ryan is an unknown politician with a deeply unpopular plan. Right now, the conventional wisdom is that this somehow means he was a smart pick who can help Romney win voters who are looking for “big” solutions to our problems.