The blue wave was big — and significant — in state legislatures

In the next Congress, Republicans will no longer control both houses of Congress, setting up two years of partisanship on Capitol Hill. (Video: Meg Kelly/The Washington Post, Photo: Evelyn Hockstein/The Washington Post)

The 2018 election gave Democrats control of the House of Representatives, illustrating once again that the president’s party nearly always loses seats in midterm elections.

But Democrats gained seats in state legislatures, too. How does that success compare with the past? And what was driving it?

The graph below shows the percentage of statehouse seats gained or lost by the opposition party in midterm elections over the past 100 years. Democrats improved their seat share in statehouses by 4.6 percent. This was just below the historical average for statehouses (5 percent) and also below their gains in the U.S. House (8.5 percent).

Democratic gains in state legislatures reclaimed more than 250 of the many seats lost during Barack Obama’s presidency. Democrats took functional control of an additional seven state legislative chambers, while Republicans took control of only one, the Alaska House. In addition, Democrats gained enough seats to deprive Republicans of supermajorities in Michigan, North Carolina and Pennsylvania. This will keep Republicans from having enough votes to override the vetoes of Democratic governors of those states. Democrats achieved supermajorities for themselves in seven chambers.

Democratic gains in state legislative elections reflect the important role of national politics in subnational elections. Nationwide, the larger the drop in approval of President Trump in a state since he took office, the larger the Democratic gains in the state’s legislature. The graph below shows that if a state’s approval rating of Trump had dropped by 10 percentage points, Republicans lost about 4.5 percent of their statehouse seats in that state.

The 2018 midterm election set a record for the number of single-party-controlled legislatures. When the new legislators take their seats, only the Minnesota legislature will be divided. The last time the United States had only one divided state legislature was in 1914.

These Democratic gains set the stage for future elections. A majority of the governors holding veto power over redistricting in 2021 are now Democrats. Moreover, in most states where Democrats won control of a legislative chamber on Tuesday, the legislature has a role in the congressional redistricting process — although Colorado voters took this power away from the legislature via the initiative process.

In short, Republican losses in state legislatures were typical for a midterm election, but these losses could translate into larger losses for the party in the future.

Steven Rogers is an assistant professor at Saint Louis University. Follow him on Twitter @SteveRogersInfo.