The Republican Party currently enjoys an edge. The advantage can be seen in Congress, state politics, judicial rulings, foreign and domestic policy, party finances, the media, public attitudes, and economic and demographic developments. Yet the Republicans do not seem capable of translating this into a durable electoral majority.

Conditions now exist within American politics that will facilitate the establishment of Republican rule. Many of these conditions have ripened during the past decade. They include rules governing elections and campaign finance, shifts in core...