Katie Hill, a Democratic candidate from California's 25th Congressional District, waves at supporters during an election watch party on Nov. 6 in Santa Clarita. (Marcio Jose Sanchez/AP)

This article was updated on Nov. 15.

It is perhaps more apt to describe the results of the 2018 midterm elections not as a wave in favor of the Democrats but as a tsunami. Not because of scale, though the Democratic gains in the House were substantial, but because tsunamis build slowly and steadily, water pushing inland slowly covering ever more territory. A wave hits and crashes. A tsunami just keeps coming.

There are, as of writing, two House contests in California that have not yet been resolved. In both cases, the Republicans are in a tough position. Katie Porter (D) leads incumbent Rep. Mimi Walters (R) by a narrow margin. Young Kim (R) leads Gil Cisneros (D) — but the margins as votes are counted have favored Cisneros. Both Porter and Cisneros are favored to win by FiveThirtyEight.

If or when they do, it will cap a remarkable stretch since polls closed Tuesday last week. Contested House races in California have been toppling in the Democrats' direction ever since. Even if Kim pulls out a victory in the 39th District, the Democrats will hold 44 of the state’s 53 seats — 83 percent of the total.

The last time Democrats held as high a percentage of California’s House seats, the Civil War was raging. In fact, Democrats then held all of California’s seats — or, put another way, both of them.

Using data compiled by MIT Election Data and Science Lab, we evaluated how much of each state’s House delegation either party has held since 1976. The density of each party’s control of seats after each election is shown from left to right. (Uncalled 2018 races are attributed to the currently leading party.)


(Philip Bump/The Washington Post)

There are some interesting patterns in the graph above. You can see, for example, the trend in most Southern states away from the Democratic Party over the past 40 years. Independent Sen. Bernie Sanders’s tenure as Vermont’s sole representative stands out distinctly. Massachusetts’s long record of electing solidly Democratic House delegations stands out, as well.

On Thursday, Maine’s 2nd District was called for Democrat Jared Golden. That means that there are no longer any Republicans serving in the House in New England -- and only one Republican, Sen. Susan Collins (R-Maine) -- serving in Congress from the region at all.


(Philip Bump/The Washington Post)

What’s obscured a bit in the annual data above but revealed in that second map are how many states’ House delegations are now as densely partisan as they’ve been at any point in the past 40 years. In the most recent elections, 28 states elected representatives from each party who match the highest density of one party or the other since 1976. Four states — including California — set new highs in the dominance of the Democratic Party in their congressional representation.


(Philip Bump/The Washington Post)

Here, too, there are interesting patterns. This map mirrors, to a large extent, the safest presidential states in any given year for each party. One exception is Iowa, which is redder than this would suggest. (After the 1964 elections, Iowa was more solidly Democratic than it is now, with nearly twice as many representatives.) New York is bluer, but it had a higher peak of Democratic density a decade ago.

That only 19 states aren’t at peak partisanship seems remarkable and apparently a natural reflection of the political moment. That four states are more partisan than at any point in the last 40 years itself seems remarkable.

But, then, this was a Democratic tsunami year, and Republican strongholds are still getting washed away.