The 2001–2002 round of congressional redistricting was the most incumbent-friendly in modern American history, as many pundits have noted. But the new district lines not only insulated incumbents from competition. They also froze into place a key feature of the 1990s districts that has escaped the notice of the press, political scientists, and most redistricting attorneys and experts: a “distributional bias” that gives Republicans a roughly 50-seat head start in the battle for control of Congress. In combination, these two features — extreme protection of incumbents and a powerful pro-Republican bias — might prevent Democrats from regaining control of Congress in this decade even if public opinion shifts heavily in their favor.
Sam Hirsch, The United States House of Unrepresentatives: What Went Wrong in the Latest Round of Congressional Redistricting, Election Law Journal, Volume 2, Number 2, 2003