There are two apparent causes. First, Republicans have actually been more aggressive than Democrats across the country. Like political analyst David Wasserman recently wrote for NBC News:
Thanks to voter-approved reforms, many heavily blue states employed bipartisan redistricting commissions that produced neutral or only marginally Democratic-leaning political maps, including California, Colorado, New Jersey, Virginia, and Washington. And state courts in Maryland and New York cracked down on attempts to rig Democratic legislatures.
Instead, Republicans were able to rig congressional maps to their advantage in Florida, Georgia, Kansas, Ohio, Tennessee, and Texas, among others, and the conservative-dominated U.S. Supreme Court blocked Republican orders. lower courts to draw new majority black districts in Alabama and Louisiana. In Florida alone, Governor Ron DeSantis outperformed his own Legislature to pass a map that adds four additional Republican seats.
The second cause is one that Jonathan Rodden, a political scientist at Stanford University, has explained in his book, “Why do cities lose?.” Many Democratic voters live close 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 appear to give the GOP a bigger advantage in the House than it actually has. Why not? The courts have been successful in stopping some Republican attempts at voter rigging, including in Pennsylvania and North Carolina. And the Democrats have enacted their own gerrymanders in Illinois, New Mexico and Oregon.
Recent changes in the vote, related to Donald Trump, have also reduced what was once the Republican advantage on the House map. Rural areas that were already conservative became even more conservative, leading to larger Republican margins in some House races without adding new seats for the party. At the same time, college-educated voters in the suburbs leaned Democratic, helping the party swing some districts.