[L]ots of Republicans who were running as challengers or in open seats in 2010—and then won—ran as incumbents for the first time in 2012. We know that incumbency is a powerful factor in House elections, bringing candidates greater visibility, adding to their campaign coffers, and deterring quality challengers from running. On average, an incumbent in 2012 ran five percentage points ahead of a non-incumbent candidate from the same party in a similar seat. Sixty-one seats were were decided by less than this margin.
More important, once we took incumbency into account, the apparent effect of gerrymandering vanished. That is, the ability of Republicans to retain the House majority may have been due to incumbency advantage, not new and more favorable districts.