For me, the first, last and most important question about gay marriage is: Will it help or hurt marriage as a social institution? Is it, in other words, a good idea? But for political elites a second question naturally comes close behind: What are the likely political consequences of making same-sex marriage a highly visible issue, through a federal marriage amendment, or some other means? Conventional wisdom has it that this is a difficult issue for both Democrats and Republicans. This perception is probably fueled by the reality that GOP and Democratic elites live in the same zip codes and...