Elections these days are determined in part by the swing of unaffiliated voters, which both sides closely contest. They also hinge on how strongly each party’s base turns out to vote.

The base vote can be a huge factor in elections. Consider, for instance, that in 2000 George W. Bush won 72 percent of people opposed to abortion in most or all cases. In 2004, he won 75 percent of that bloc. Moreover, turnout among this group increased from 40 percent to 42 percent of the population. Taken together, this increase among an already solidly Republican group added about 3.2 million votes to Bush’s total in 2004, more than his entire margin of victory over John Kerry.