A national conflict over redistricting has been engaged, with California at the center of the maelstrom. Texas redrew its congressional map in an effort to add five Republican-controlled seats. California Democrats have responded with Proposition 50, which would add Democratic districts in the Golden State. If approved by voters in November, Prop 50 will suspend the map drawn by California’s independent redistricting commission for the next three election cycles in favor of a map recently drawn by the legislature. In the near term, the new map would add up to 5 Democratic representatives and reduce the number of competitive races; the longer-term impact is less certain.

One way to understand the Prop 50 plan’s potential impact is to look at recent presidential results, which have been a remarkably accurate gauge of US House votes in each district. If we were to assume that every district won by Kamala Harris in 2024 would go to a Democratic House candidate, the party would win 47 seats under the proposed plan, compared to 41 under the existing plan. (Democrats actually won 43 seats under the existing plan in 2024, winning in two districts that voted narrowly for Trump.)

The plan would replace 6 of the 12 overwhelmingly Democratic districts—where Harris won with 30% to 40% margins—with districts that are still solidly Democratic but with relatively more Republican voters. The Democratic voters who used to be part of these districts would then be able to help make other districts Democratic: 18 districts now have margins between 10% and 20% for Harris, up from 7 in the current plan.

The current plan is notable for its large number of competitive districts—13 had Harris vote shares between 45% and 55%, with 6 leaning Republican and 7 leaning Democratic. The Prop 50 plan cuts that number to 9, with all but one now leaning toward Democrats. The near absence of competitive Republican seats would put a ceiling on Democratic gains. Between 2012 and 2024, the Democratic share of the US House vote in California ranged from a low of 57% in 2014 to a high of 66% in 2018, and was about 60% in 2024. In a rough simulation of the 2018 election, the party would win 48 seats, only one more than under the existing plan. That would amount to a gain of 5 seats over the number the Democrats hold now. Ironically, in a more Republican climate, Democrats would fare better relative to the status quo: a 2014 simulation gives Democrats 43 seats under the Prop 50 plan, compared to just 36 under the current one.

These gains assume a close alignment between presidential and congressional voting; the more they diverge, the smaller the expected gains. Indeed, a model of recent elections that measures and incorporates typical electoral uncertainty predicts fewer Democratic wins under both the current and proposed plans, and a smaller difference between the two. The consequence of this uncertainty is smallest in a good Democratic year—in which the Democrats would still be expected to win 48 seats—and largest in a good Republican year like 2014, when the Democrats might win 38 or fewer.

Because the sitting president’s party usually loses votes in midterm House elections, 2026 is likely to look more like 2018 than 2014. But the plan would be in place for three election cycles, making the full range of possible outcomes more realistic. In short, the Prop 50 plan will probably give the Democrats most of the seats they are looking for in 2026, but with murkier consequences beyond that.

Though partisanship is the main driver of this mid-decade redraw, the plan’s partisan impact is not the only important consequence to consider: the redistricting commission’s plan factors in representation for voters of color, the shapes of the districts, and the cohesion of cities and counties within each district. Stay tuned for an analysis of the Prop 50 plan’s potential impact on these outcomes.