Mr. Adler is a computational research specialist at Princeton University’s Princeton Gerrymandering Project. Mr. Thompson is the graphics director for Opinion.

Mr. Adler is a computational research specialist at Princeton University’s Princeton Gerrymandering Project. Mr. Thompson is the graphics director for Opinion.

The midterms saw a “blue wave” of Democratic support, with the party gaining at least 26 House seats and winning the popular vote by seven points. While it was a significant victory that gave control of the House to the Democrats, they could have won even more seats if not for gerrymanders – carefully manipulated district maps that have given Republicans a substantial advantage in House elections since 2012.

Particularly in four states – Ohio, Michigan, North Carolina and Texas – Democratic candidates gained support from voters relative to 2016, but gained relatively few seats.

The popular vote increased for Democrats in these states ... ... but they gained no seats Ohio 42.5% +5.4 pct. pts. Dem. 2018 4 seats Dem. 2016 +0 seats Dem. 2018 North Carolina 46.4% Dem. 2016 +2.9 pct. pts. Dem. 2018 3 seats Dem. 2016 +0 seats Dem. 2018 Note: Popular vote reflects average of two-party vote share across districts. While the popular vote increased for Democrats in Ohio and N.C., the number of seats did not Ohio Popular vote 42.5% Dem. 2016 +5.4 pct. pts. Dem. 2018 Seats 4 seats Dem. 2016 +0 seats Dem. 2018 North Carolina Popular vote 46.4% Dem. 2016 +2.9 pct. pts. Dem. 2018 Seats 3 seats Dem. 2016 +0 seats Dem. 2018 Note: Popular vote reflects average of two-party vote share across districts.

In Ohio, Democrats Couldn’t Translate Votes to Seats

The Democratic disadvantage is most glaring in Ohio: While Democrats won 48 percent of the popular vote there, up from 43 percent in 2016, they collected only 25 percent of the state’s House seats – the same four seats they have held since 2012.

Contrast these results against a fairer system in which either party can capture, say, 11 seats if it wins 57 percent of the vote. In that scenario, the 2016 election results could have looked like this: evenly spaced, where each additional seat requires the same overall vote gain by either party.

Democratic vote share

But that’s not what happened. The results were tilted toward Republicans.

When the Republican-controlled State Legislature and Republican governor redrew the congressional map in 2011, they distorted how voters are distributed. They packed voters who tended to vote Democratic into four districts (the Third, Ninth, 11th and 13th). The remaining left-leaning voters were scattered among across a large number of districts in which Republicans won by safe margins.

The map was still in effect this year. The gerrymandering advantage built by Republicans withstood the blue wave.

Democratic vote share

The gerrymander was drafted with large enough margins that even the five-point shift toward the Democrats this year failed to push a single Republican district into the blue, leaving the 12–4 seat split intact.

Gerrymandering in North Carolina, Michigan and Texas

Republican gerrymanders in North Carolina, Michigan and Texas also withstood the blue wave, with few or no districts flipping in the Democrats’ favor:

In North Carolina, the Democrats gained 2.9 percentage points, but flipped no seats...

Democratic vote share

In Michigan, Democrats flipped two seats, but got only 50 percent of the seats despite winning 54 percent of the vote...

Democratic vote share

In Texas, the blue wave pushed almost every district toward the Democrats (including a major upset in the 32nd District), but a large number of districts just barely fail to cross the 50 percent mark…

Democratic vote share

Redistricting Reform Makes Progress

After decades of blatant partisan gerrymandering, things may finally be changing. In May, Ohioans voted overwhelmingly to put redistricting in the hands of a bipartisan commission rather than self-interested legislators, and, on Tuesday, voters also approved reform measures in Colorado, Michigan and Missouri. (Utahns voted on a measure, too, but as of Wednesday afternoon the vote was too close to call.)

Colorado: Independent Commission for Congressional Redistricting Answer Votes Pct. Yes 1,300,912 70.8 % 1,837,017 votes, 81% reporting (2,597 of 3,219 precincts) Missouri: Revise State Redistricting Answer Votes Pct. Yes 1,459,576 62.0 % 2,354,588 votes, 100% reporting (3,256 of 3,256 precincts) Michigan: Create Redistricting Commission Answer Votes Pct. Yes 2,449,302 61.1 % 4,005,445 votes, 98% reporting (4,710 of 4,808 precincts) Utah: Redistricting Commission Answer Votes Pct. For 368,248 50.3 % 732,449 votes, 74% reporting (1,836 of 2,492 precincts)

A Fairer Future

The congressional district maps will next be redrawn in 2021. If voters continue to demand redistricting reform, those maps should be much fairer and better reflect their interests.

They may look similar to California and Arizona, two states where the maps were drawn by independent citizens commissions – the gold standard of redistricting.

Democratic vote share

Democratic vote share