Political leaders have less power “than ever before” to reward and protect party loyalists “who take a tough congressional vote . . . or who dare cross single-issue voters and interests,” writes Rauch. Once, those powers were considerable. Parties selected candidates for office and funded their campaigns. In Congress, committee chairmen could fashion controversial legislation behind closed doors.

All these powers have been curbed. Candidates nominate themselves by running in primaries; they become free agents. Contributions to candidates and parties are limited by law; this has inspired “independent” groups, outside the candidates’ and parties’ direct control, that provide substantial campaign funding. Committee meetings must generally be open.

The new political system favors ideological extremes. “Inside their gerrymandered districts,” Rauch argues, “incumbents are insulated from general-election challenges that might pull them toward the political center, but they are perpetually vulnerable to primary challenges from extremists who pull them toward the fringes.” Ideological “purity” trumps pragmatism. Technology reinforces the bias. In the Web and cable-news era, politicians constantly need to reassure their constituents that they haven’t sold out.