Gerrymander, undone*

The new plan that brings representative, competitive government to Wisconsin for the first time in over a decade. *In the Senate, restrictions apply.

February 23, 2024: updated to include odd-numbered Senate districts that might need an election this year.

Today, the most extreme legislative gerrymander in the nation was undone. The Wisconsin Supreme Court’s current redistricting case led to submission of six sets of remedial maps for the court’s consideration. But rather than wait for the Court to choose among them, the legislature and Governor Tony Evers agreed on his submission, and today he signed it into law.

Faced with the certainty that one of the maps would be adopted by the Court, the Republican-controlled General Assembly took matters into its own hands. I can identify two plausible reasons why they did so: (1) It leaves them a little more residual partisan advantage than the alternatives, and (2) it guarantees them two more years of control in the Senate, assuming that the Wisconsin Supreme Court takes no further action.

A few weeks ago I reported a deep dive by the Princeton Gerrymandering Project/Electoral Innovation Lab into the newly enacted proposal. I will use that analysis to put today’s news into a larger context.

The General Assembly makes the best of it

There’s been some discussion of a possible trick hidden within the General Assembly’s action, such as a hidden avenue for filing a new lawsuit. That concern might be overblown. There is so much distrust in Madison that counteroffers are met with suspicion. A simpler explanation is that the Governor’s plan was the best offer on the table, and they wanted to lock it in.

Consistent with our previous analysis at the Electoral Innovation Lab, court-appointed special masters concluded that four of the litigants submitted plans that met the court’s criteria: Governor Evers, the Wright petitioners, Senate Democrats, and the Clarke/LawForward petitioners. In our analysis, the Wright map and the Law Forward map would have been most evenhanded between the major parties. The Evers proposal contains some residual Republican advantage. That’s the first way in which the General Assembly made the best of an unfavorable situation.

A (mostly) majoritarian Assembly in 2024

One simple measure of treating the parties equally is the difference in partisanship between the median district and the statewide average. This is called the “mean-median difference,” and it’s an effective way to measure partisan bias in a 50-50 state like Wisconsin. It is simple to calculate and has a clear interpretation: If the mean-median difference is zero, then half the districts are above average, and whoever gets a majority of the vote should also win a majority of the legislature. As the mean-median difference deviates from zero, one party develops a substantial advantage.

The mean-median difference is closely related to the expected number of seats. Here is the relationship for the Assembly:

As you can see, compared with the other compliant submissions, the Evers plan is closer to the Law and Liberty and Legislative GOP plans, which the special masters identified as partisan gerrymanders.

Another way to interpret the mean-median difference is to ask: what’s the probability that a party who wins a majority of the statewide vote will also win a legislative majority? We can do that by assuming that each party’s statewide vote share will vary somewhere within the range of 46% to 54%.

In Wisconsin, because of the clustering of Democratic and Republican voters in different regions of the state, one might expect it to be difficult to simultaneously maximize partisan symmetry and minimize geographic splits. However, Governor Evers came close with mean-median differences of 1.7% (104 splits) in the Assembly and 1.0% (68 splits) in the Senate. For these reasons, the Princeton Gerrymandering Project gives an A grade to both the Assembly and Senate plans from Governor Evers.

Let’s translate this into actual election performance.

In the Assembly, if Republicans get more votes, they will win a majority 95% of the time. If Democrats get more votes, they will win a majority about 60% of the time.

The Senate plan is better: if Republicans get more votes, they will win a majority more than 90% of the time. If Democrats get more voters, they will win a majority about 75% of the time.

That is pretty symmetric. However, in the Senate, there’s a catch.

A majoritarian Wisconsin Senate…but not until 2026

The Wright petitioners’ plan had superior partisan symmetry in both Assembly and Senate. In the Senate, their plan had a second advantage: it leveled the playing field immediately. The reason is that even-numbered districts all come up for election in 2024. According to their own analysis, those districts were arranged so that the chamber as a whole would become majoritarian in that first election.

Governor Evers does not make this claim for his plan. In the Evers plan’s even-numbered districts, 8 of them lean Democratic by seven points or more, 6 districts lean Republican, and two are competitive and could go either way. There is one more Democratic-leaning odd-numbered district (SD17) that may have to have an election (see below). Because it takes two cycles for the entire Senate to turn over, the Senate under his plan will likely have 17 to 19 Republicans (out of 33 districts total) after the 2024 election, even if they lose the statewide vote. And the Senate will not become fully majoritarian until 2026. So Republicans will likely hold on to their Senate majority for at least two more years.

The Wisconsin Supreme Court could conceivably require that the new Senate map be used to elect all 33 senators this November. At a minimum, Wisconsin law (Wis. Stat. § 17.03(4)(b)) says a legislative office is vacant when the “incumbent ceases to be a resident of … the district from which elected,” which seems to imply that some of the odd-numbered districts that would normally come up in 2026 might become open for this November’s election. For now, that potentially includes Senate districts 1, 17 and 19, as documented in a brief from the Legislature. This depends in part on whether incumbents who live within the old boundaries of those districts choose to move. Finally, redistricting has been said in the past not to create Senate vacancies, but the Attorney General could weigh in on that.