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.