How Many Seats in Congress Is Control of Redistricting Worth?

How large a benefit is partisan control of the redistricting process? Do legal constraints on redistricting—such as the preclearance requirement of the Voting Rights Act—alter this benefit? Are institutions designed to reduce the benefit to partisan control—such as redistricting commissions—effective? To measure the effects of partisan districting on the electoral fortunes of the parties, we collect data on the partisan composition of state government, House election outcomes, and moderating institutions over an 80‐year period. Our results suggest that over time, both parties have benefited from unified control, with the effects largest in states where voters were evenly divided among the parties and smallest in states where the controlling party had a large advantage in the electorate. The effects have changed over time, with both parties having equally benefited from control during the middle of the 20th century, the benefit largely disappearing in the late part of the century, and the Republican Party seeing a moderate advantage from control in the current century. The benefits of partisan control were not diminished in states with redistricting commissions. The preclearance requirement appears to have hurt the Democratic Party except when its vote share was very low. The aggregate effects of partisan redistricting are moderate in magnitude—in the modern period, this effect has typically been less than 10 House seats, with the last election where control of the House would have flipped in expectation occurring in 1954.