Democrat Conor Lamb’s apparent victory in the special election in southwestern Pennsylvania’s 18th congressional district—a 627-vote margin Republicans say they may contest—is a perfect illustration of how big 2018’s blue wave has to be to surmount the swing-state barriers created by the GOP.

Lamb’s district is heavily gerrymandered, meaning voters have effectively been segregated by party. That happened in 2011 in Pennsylvania when a GOP-run legislature and governor monopolized the mapmaking process. The district begins in the state’s southwestern corner, but has a jigsaw-puzzle-like piece jutting outward to the east.

Those political boundaries ensured that in normal election years, like most of this decade, there would be anywhere from 6 to 8 percent more reliable Republican voters casting ballots than Democrats. Of course, 2018 is not a normal election year. (The district also will evaporate after this race, because the state’s Supreme Court just threw out that 2011 map as unconstitutional, and drew new lines.)

Lamb’s apparent majority came from a mix of enthusiastic Democrats turning out and a handful of independents and Republicans voting for him as well. Some Republicans also stayed home, disillusioned by Trump and a GOP candidate all but mimicking the president. It’s worth paying attention to the size of the Democratic turnout. As academic experts noted following the vote, Lamb’s margin of victory might be as much as Democrats can expect in 2018.

In short, Democrats had a 15 percent higher voter turnout rate than Republicans, but due to gerrymandering, they barely crossed the 50 percent vote count line. That’s the structural advantage redistricting produces, and it remains in play for 2018 House elections in states like Florida, Georgia, Ohio, Wisconsin, Michigan and North Carolina—all red-run states with otherwise purple voter demographics.

The University of Florida’s Michael McDonald, a nationally respected gerrymandering analyst and voter turnout expert, mused on Twitter that Lamb’s showing was about as much as Democrats could hope for this year.

“Stats on how many districts are less Democratic than #PA18 miss an important point. Elections are noisy. Better question to ask: where is #PA18 in the distribution of winnable seats for the Democrats? Is it at the tail end, or can Democrats win seats even more Republican?” he tweeted.

“From the looks of it, it’s the tail end. Dems had a +15 turnout advantage and hit a statistical tie,” replied d_mcc, noting the blue wave’s size.

“I think this is right,” McDonald replied. “#PA18 is about as far as the Democrats can reach. Doesn’t mean they shouldn’t run candidates elsewhere in case Republicans nominate a bad candidate, but I expect Democrats to concentrate on less Republican-leaning seats come November.”

So Democrats had a 15 percent turnout advantage and basically ended up with a statistical tie. That’s striking but mirrors the results of Virginia’s House of Delegates election last fall. A quarter-million more Democrats voted statewide than Republicans, good for a 9.5 percent turnout advantage, in a state with Republican-gerrymandered congressional maps.

The majority to control Virginia's lower legislative chamber turned on one race that ended in a tie. To decide it, a name was drawn out of a bowl going to the Republican incumbent, thus keeping that legislative majority red. This structural advantage is indicative of what Democrats (or independents) face this year in gerrymandered states if they want to end Republican rule.

The PA-18 race showed that 2018’s blue voter turnout wave is going to have to be a big one—with much higher percentages of Democrats voting—in order to win. This is impact of extreme gerrymanders, predating who the candidate is, whether the candidate is a centrist or progressive, or how the district’s voters feel about the president and the party enabling him.