There are two apparent causes. First, Republicans really have been more aggressive than Democrats nationwide. As the political analyst David Wasserman recently wrote for NBC News:
Thanks to reforms passed by voters, many heavily blue states employed bipartisan redistricting commissions that produced neutral or only marginally Democratic-leaning political maps — including in California, Colorado, New Jersey, Virginia and Washington. And state courts in Maryland and New York struck down Democratic legislatures’ attempted gerrymanders.
By contrast, Republicans were able to manipulate congressional maps in their favor in Florida, Georgia, Kansas, Ohio, Tennessee and Texas, among others, and the conservative-dominated U.S. Supreme Court blocked lower court orders to draw new Black majority districts in Alabama and Louisiana. In Florida alone, Gov. Ron DeSantis overpowered his own Legislature to pass a map that adds an additional four G.O.P. seats.
The second cause is one that Jonathan Rodden, a political scientist at Stanford University, has explained in his book, “Why Cities Lose.” Many Democratic voters live packed tightly together in cities. As a result, even Democratic state officials often struggle to avoid drawing districts where Democratic House candidates win landslide victories, effectively wasting votes.
In 2020, only 21 Republican House candidates won their elections by at least 50 percentage points. Forty-seven Democrats did.
226 to 209
If anything, these two factors would seem to give the Republican Party a larger advantage in the House than it actually has. Why don’t they? Courts have managed to halt some Republican attempts at gerrymandering, including in Pennsylvania and North Carolina. And Democrats have enacted their own gerrymanders in Illinois, New Mexico and Oregon.
Recent shifts in the vote — connected to Donald Trump — have also shrunk what once was the Republicans’ advantage in the House map. Rural areas that were already conservative became even more so, leading to bigger Republican margins in some House races without adding any new seats for the party. At the same time, college-educated voters in the suburbs swung toward the Democrats, helping the party flip some districts.