Politics 101 suggests that you play toward the center of the electorate. Although this rule has more frequently been violated when it comes to vice-presidential picks, there is evidence that presidential candidates who have more “extreme” ideologies (closer to the left wing or the right wing than the electoral center) underperform relative to the economic fundamentals.

Various statistical measures of Mr. Ryan peg him as being quite conservative. Based on his Congressional voting record, for instance, the statistical system DW-Nominate evaluates him as being roughly as conservative as Representative Michele Bachmann, the controversial congresswoman of Minnesota…

Can Mr. Ryan help Mr. Romney to carry his home state, Wisconsin? Possibly, but Mr. Romney is quite far behind in the polls there. And although Mr. Ryan has won by impressive margins in his home district, his popularity statewide is mixed, with 38 percent of voters having a favorable impression and 33 percent an unfavorable one throughout Wisconsin…

I think there are other “bold” picks that Mr. Romney could have made — Senator Marco Rubio of Florida, for instance, or Gov. Chris Christie of New Jersey or Gov. Brian Sandoval of Nevada — where the balance of risk and reward would have been a little better. Some of these candidates, especially Mr. Rubio and Mr. Christie, would also have excited the Republican base. But they might also have had a more natural appeal to independent voters, and to demographic groups that Mr. Romney is struggling to win over.