Polarization and gridlock are defining characteristics of present-day American national politics, reflecting the interaction of the Madisonian system's checks and balances with the widening partisan and ideological divisions in the House and Senate. These divisions have coevolved with complimentary changes in electoral politics over the same period in a mutually reinforcing spiral. The partisan, ideological, and policy opinions of American voters have grown more internally consistent, more distinctive between parties, and more predictive of voting in national elections. Party loyalty has increased, ticket splitting has decreased, and winning congressional seats against the local partisan grain has become much more difficult. The congressional parties thus represent increasingly divergent electoral constituencies. Republicans enjoy a structural advantage in the distribution of their regular voters across House districts, so these changes have solidified their control of the House even as their national party grows less competitive nationally. Divided government and polarized politics thus rest on a firm electoral base, and partisan warfare in Washington is unlikely to diminish until voters begin to punish the warriors.