IN MOST DEMOCRACIES, nonpartisan technocrats write voting rules. In America, however, they are politically controversial. For decades, Southern states used shameful “Jim Crow” laws, such as literacy tests, to stop black people from voting. Such abuses continued until Congress banned them in the Voting Rights Act (VRA) of 1965.

In recent months a new battle over voting rules has begun. Republican legislators in 47 states have proposed reforms changing when and how people vote, citing electoral security. Democrats fear these bills will make voting harder for non-whites.

Such warnings may be justified in some cases. However, studies of voting requirements such as a photo ID have not found that they provide big partisan advantages. Instead, the redrawing of Congressional districts is likely to have a bigger impact.

Both parties gerrymander (drawing legislative borders to maximise their seats). However, Republicans run more state governments, and a greater share of Democratic states entrust redistricting to nonpartisan panels. This means Republicans could gerrymander up to 187 House seats for 2022 compared with 75 for Democrats.

Beyond the constitutional rule that laws protect everyone equally, federal law places just two limits on gerrymandering. First, districts must have similar numbers of people. Next, in 1986 the Supreme Court applied the VRA to ban states from diluting non-whites’ impact in places where they vote as a bloc. If mapmakers split up areas with lots of minorities—who tend to support Democrats, and often elect candidates of their own race—into too many districts, they risk having such lines redrawn.

The judiciary has chipped away at the VRA. In 2013 the Supreme Court threw out a rule requiring federal approval of changes to state voting laws. This month the court will decide if new voting policies in Arizona violate Section 2, which bans electoral laws with discriminatory effects and is the basis for the VRA’s limits on redistricting. If the court narrows its interpretation of this clause, it would further weaken the law.

Any estimate of how parties’ strength in Congress would change without the VRA will be inexact. But on current trends, it is an increasingly relevant exercise. At our request,Stephen Wolf, a districting expert, went through every state where Republicans control redistricting and drew the most Republican-friendly, VRA-compliant gerrymander he thought was plausible. He then repeated this process without regard for the VRA, and tested whether that let him flip even more Democratic seats.

Outside of the South, the VRA had little effect. Most of the Republican gains came from redistricting suburbs that contain lots of college-educated whites, not from “cracking” minority areas. However, without the VRA, Alabama, Louisiana and Mississippi went from one Democrat each to all-Republican slates. In total, the VRA lowered the number of seats that Republicans added from 15 to nine.

This analysis shows that the Supreme Court could help Republicans win a few seats by curtailing the VRA. It also shows that far bigger changes could result from new limits on gerrymandering, like those in one bill currently before the Senate.■

Sources: Stephen Wolf, Daily Kos; IPUMS NHGIS, University of Minnesota;The Economist