But it is undeniable that redistricting played a role as well. The new lines helped Republicans maintain their control of the House, largely because they were able to add more Republican voters to districts where Republicans won close races in 2010.
Michael P. McDonald, an associate professor of public affairs at George Mason University who has served as a consultant on redistricting for both parties, said there was a reason both parties fight so hard for the power to draw the maps — noting that they were not going to all that trouble “just to draw neutral plans.”
Democrats also drew gerrymandered lines in states where they controlled the process, but had less of an impact over all because they had control in fewer states, said Keesha Gaskins, a senior counsel at the Brennan Center for Justice, which has been studying the impact of redistricting.
In Illinois, where Democrats drew the maps, Republican Congressional candidates won 45 percent of the popular vote but only a third of the House seats. And in Maryland, Republicans won 35 percent of the votes but just 13 percent of the seats.
An analysis by the Brennan Center found that the new lines that took effect this year may have changed which party won in at least 25 House districts this year, and that they helped Republicans win a net gain of six more seats than they would have won under the old maps.
One particularly striking finding in their analysis highlights the power that comes with drawing the maps. In states where Republicans controlled the process, it found, their candidates won roughly 53 percent of the vote — and 72 percent of the seats. And in the states where Democrats controlled the process, their candidates won about 56 percent of the vote and 71 percent of the seats.
