IT WOULD BE easy for those complacent about partisan redistricting to look at this year's election as evidence that it's not a problem after all. Gerrymandering of congressional district lines failed to protect a number of incumbents around the country from defeat. It didn't prevent the House of Representatives from switching hands. Maybe letting politicians use sophisticated computers to choose their own voters actually calcifies the American electoral system less than the past few elections have suggested.

The election this year was dramatically more competitive than others in recent history. According to data from the Center for Voting and Democracy, 34 House races were decided by a margin of fewer than 5 percentage points, more than in 2002 and 2004 combined. An additional 26 were decided by fewer than 10 percentage points, compared with 28 in the past two elections combined. The reason for the change is simple: Voters were angry at Republicans. And while partisan redistricting can make red districts redder and blue districts bluer, it cannot stop either from going purple as a party fails to persuade its own voters or get them to show up at the polls.

But don't let these numbers fool you. The overwhelming majority of seats remained beyond serious democratic competition -- as always. Three hundred and seventeen seats were decided by margins of greater than 20 points, 95 of them by more than 40 points. In 55 races, candidates of one major party faced no opposition from the other major party. In other words, this year's competitiveness is only comparative. For most members, incumbency still offered surefire protection from voter dissatisfaction. And the power of incumbency undoubtedly protected Republicans from further losses. Of the closest races in which incumbents hung on to win, only two, at most, involved Democratic incumbents scraping by, both in Georgia. The rest were all Republicans, several of whom clearly benefited from the way the lines had been drawn after the 2000 Census.

Partisan redistricting is not the only -- or even the principal -- factor shielding incumbents from electoral accountability. Even districts drawn in an apolitical fashion will tend to have stronger ideological centers of gravity than one might imagine. And incumbents also benefit from fundraising advantages and the ability to bring home pork. But redistricting adds to all of that a measure of simple cheating to make challenges more difficult. The party in power in a given state gets to shield its own officeholders and burn the other side's, or the parties can jointly conspire to protect virtually all incumbents. As this election shows, it doesn't always work completely. America is not a Soviet republic. But even at their most competitive, House races are far from models of electoral accountability.