John McCain won the Republican primaries by running as a pro-life Reagan Republican, committed to low taxes and a strong defense. We assume that he will therefore pick a running mate who shares his positions on fundamental issues whom conservatives can support with enthusiasm. We assume, as well, that he is more interested in winning the White House than in redefining the Republican party, and will not try to make the party platform reflect each of his views. Win or lose, in picking a running mate Senator McCain will also be conferring front-runner status on a candidate for his party’s...