Many Democratic voters and activists are giddy about their party’s chances of retaking the House in 2018. Savoring surprise victories in Alabama, Pennsylvania and Virginia, they have visions of a blue wave election.

If political history were the only guide, they would be right. Polls in recent days have shown Democrats with around a 9-point average lead on the generic congressional ballot, which asks Americans which party they’ll vote for in the coming election. Under ordinary conditions, a lead that size would be more than enough to net the 24 seats Democrats need to regain a majority.

But a big reality check is in order. Even the strongest blue wave may crash up against a powerful structural force in American politics: extreme gerrymandering. Pending court cases, including one scheduled to be argued before the Supreme Court on Wednesday (in which our organization filed an amicus brief), may change the terrain going forward. But no matter how the high court rules, its decision will almost certainly come too late to affect the 2018 vote.

That means most Americans will participate in this year’s elections under gerrymandered congressional maps that were created in the wake of the 2010 Tea Party wave. Nearly a decade later, those gerrymanders are still tilting the House toward Republicans, particularly in a few key states.