There is an increasing chance that in a Democratic wave election, gerrymandering could backfire on the Republicans, and lead to a massive Democratic sweep, with a pickup of 75 seats or even more. (It takes only 24 for Dems to take back the House.)
Here's how that works. Let's say you are the Republican architects of extreme gerrymandering in Ohio, Pennsylvania, or Wisconsin. You redraw the districts, as Republicans did in 2012, so that a popular statewide vote of 52-48 Republican translates into an allocation of two-to-one Republican House seats.
In order to accomplish that trick, however, you need to spread out likely Republican voters. You assume a normal election, with a modest but not an overwhelming Republican margin in each district.
But in the case of a Democratic wave election, the tactic backfires and the wave turns into a tsunami, because there aren't enough Republican votes to go around. An 18-point average advantage for Democrats, depending on how the votes are distributed, could turn dozens of gerrymandered Republican seats into Democratic ones. That, plus normal Democratic gains in non-gerrymandered districts, could make 2018 one of the tidal swing years.
It's true that Republicans will try to steal elections by voter suppression tactics, but that only operates in some states, and can only take you so far. The ordinarily risk-averse Democratic Congressional Campaign Committee now considers fully 91 Republican-held seats worth contesting. The wave that began on Election Day in Virginia, and deepened with the Alabama election of Doug Jones, compounded by voter backlash against the tax bill, will only intensify.