Many in both parties are expecting a “blue wave” this November, as angry liberal voters flock to the polls and sweep Democrats into office. Republican incumbents are retiring at an unusually high rate; the latest is Rep. Ryan Costello of Pennsylvania, who just announced that he won’t seek reelection, because like many others in the GOP, he apparently believed the environment is brutal for his party.

But a new report from the Brennan Center for Justice suggests that Republicans yet may have reason to hope, anyway. According to the authors, the deck of district lines is so dramatically stacked in the GOP’s favor that it will take an absolutely historic wave for the Democrats to win control of the House.

This report takes a slightly different approach from many other examinations of gerrymandering, by asking what it would take in each state for Democrats to flip one additional seat. In some places where Republicans have aggressively gerrymandered, it’s a steep mountain to climb. In Ohio, where Republicans now have 12 of the state’s 16 seats, Democrats would have to improve on their statewide performance from 2016 by 13 points, going from winning 42 percent of the vote to 55 percent, just to expect to get one extra seat. In North Carolina, where Republicans control 10 of the 13 seats, Democrats would have to do 5 points better to get one more seat.

“A big reality check is in order,” the Brennan scholars write in a New York Times op-ed. “Even the strongest blue wave may crash up against a powerful structural force in American politics: extreme gerrymandering.”

According to their analysis, Democrats would have to win the national popular House vote by 11 points in order to expect to overcome the GOP advantage and gain more than the 23 seats they need to win control of the House. Other models have put the required Democratic advantage smaller: Emory University political scientist Alan Abramowitz argues that a popular vote margin of 4 to 5 points could be enough for Democrats to gain a House majority.

But whatever the actual number is, on a basic level of democratic responsiveness, this is spectacularly unfair.

After all, while every district is different and the national popular vote is never going to perfectly mirror the number of seats each party wins, at the very least it should be close. If one party wins the vote by a significant margin, but the other party controls the chamber, the system is failing to represent the voters. (Similarly, the electoral college warps the system in favor of Republicans when in two of the past five presidential elections their candidate can get fewer votes than the Democrat but still wind up president.)

President Trump's health-care actions could have ripple effects throughout the Affordable Care Act's marketplaces. (Jenny Starrs/The Washington Post)

But while it may seem fundamentally unfair for one party to get far more seats than their vote total would suggest, it’s actually not unusual. In fact, according to this analysis from the Brookings Institution, since 1946 the party controlling the House has always gotten a “seats bonus” averaging 5.6 percentage points (the difference between the proportion of seats they control and the proportion of the popular House vote they won). Democrats were particularly angry about the fact that in 2012 they won 51 percent of the popular vote but only 46 percent of House seats. But it wasn’t out of line with previous history, except for the fact that the difference determined who controlled the chamber, because it enabled Republicans to keep control. For many years, that seats bonus benefited Democrats, and these days it benefits Republicans.

But over time, the reasons one party gets a bonus, and what it would take to reverse it, can change. Right now the GOP’s bonus is built on two things: political geography and gerrymandering. The political geography part comes from the fact that Democratic voters are disproportionately concentrated in cities with large populations, which produces a lot of “wasted” votes (winning each election by way more than you need to, while it would be more efficient to distribute your voters around to give you a better shot in more districts).

The other part of the story — the one that can be more easily altered — is gerrymandering. After 2010, a big Republican year, the GOP used their newfound control of the post-census redistricting to solidify their position. The Supreme Court could well rule that extreme partisan gerrymandering is unconstitutional — they’ve already heard (but not yet decided) a case about state legislative lines in Wisconsin, and this week they’ll hear a case about a Democratic gerrymander of lines in Maryland. While it’s hard to tell how they’ll rule, the Democrats’ best hope is to gain control of some GOP-controlled legislatures in the 2018 and 2020 elections, then use that control to redistrict more in their favor after the 2020 Census. “More in their favor” need not be partisan redistricting; it could mean shifting to a system with a nonpartisan commission drawing the lines, as some states already have. Since the maps in so many states are gerrymandered in Republicans’ favor, even a switch to an equitable map would represent a major victory for Democrats.

That’s the future, though. What about this year? I’d suggest that even if the Brennan analysis is worthwhile, there are still reasons to believe that the Republicans could be facing catastrophe. That’s because as well-gerrymandered as many states are, there’s a tipping point that could be reached with a significant Democratic wave.

What I mean is this: The Brennan report asks, what would it take to flip one more seat in each state? The answer is a big Democratic advantage. But once you’ve reached that one-seat gain, marginal improvements in vote share start yielding bigger gains. Let’s take North Carolina, where Republicans control 10 of the 13 seats. According to the Brennan analysis, Democrats would have to win 53 percent of the statewide House vote to put just one more seat in play. But if they won 55 percent of that statewide vote — about as well as they did in 2008 — they’d be on track to win four more seats and control a majority of the state’s House delegation.

That doesn’t mean that the system is fair. But if things keep going the way they have been, Democrats might be able to produce a big enough win that the House will wind up actually representing the will of the electorate. If not, it will mean the firewall that Republicans built against genuine majority rule held firm again.