Yet here’s a surprising fact: Even a wave as powerful as those in 2006 or 2010 might not give Democrats control of the House.
They need only 23 seats to make that happen, which doesn’t seem like a lot. For comparison, in 2006 Democrats picked up 31 seats, and in 2010 Republicans picked up 63.
What has changed is that the Republican caucus has fled to higher ground.
Today, there are just nine Republicans who represent districts that tilt toward the Democrats, based on the districts’ voting in the last two presidential elections compared with the country as a whole. There were 24 Republicans in such a precarious position in 2006, 67 such Democrats in 2010 and 90 Democrats in 1994.
In a wave election, Republicans representing Democratic-tilting districts like these would be projected to lose their re-election bids. They are the figures farther down the slope in the accompanying illustration. (The one all by itself at the bottom, representing Pennsylvania’s Fifth, was sunk by redistricting.)
Mostly, though, the districts are above sea level. Incumbents who represent even somewhat Republican-leaning districts are generally favored to win re-election, even in a wave election. Waves aren’t necessarily as deadly as you might think.
This makes it hard to imagine huge Democratic gains, like those the Republicans made in 2010. In past elections, incumbency has been worth around seven points, although that advantage has been declining.
Republicans have so many safe seats that the Democrats would be expected to gain only 22 seats if they flipped Republican-held districts at a rate equivalent to the waves of 2006 or 2010 (without factoring in the large number of open seats). It would leave them one seat short of a majority.