Next week, the Supreme Court will hear oral argument in Gill v. Whitford, a case challenging Wisconsin's legislative district lines as an unconstitutional Republican gerrymander. It's attracted attention because many high-minded commentators have blamed partisan gerrymandering for today's highly polarized politics -- and for the fact that Republicans have won majorities in 67 of the 98 houses of state legislatures and in 10 of the past 12 elections in the U.S. House of Representatives. But as I and others have argued, gerrymandering has contributed only marginally to Republican success. More important is demographic clustering. Democratic voters are heavily clustered in...