The mantra “past performance is no guarantee of future results” applies to many things, including trying to predict election results ahead of time. But how many voters in a deep blue state showed up in this month’s primary – and how they voted – may provide insight into the political environment in November.

Election data analysts point to Washington state’s primary results as a useful barometer of which party is likely to win the national popular vote for the House of Representatives and to what degree.

Here’s how to do it: add up the total votes for Democrats and Republicans in Washington’s 10 House district primaries. Then, take the difference between them and subtract 12 to get a rough estimate of the predicted House margin in November.

An NPR analysis of the Washington primary using this framework estimates that Democrats could win the national popular vote by about 4 points, which is slightly more than the 2020 presidential election that saw them narrowly win the White House and control of both chambers of Congress.

In other words, it will likely be yet another close race.

This year, Democrats won about 57.3% of the Congressional primary vote to Republicans’ 41.3%, or a margin of 16%. Subtract 12 percentage points, and you get that estimate of a 4% popular vote victory for Democrats later this year.

Current presidential polling averages have Vice President Harris leading former President Donald Trump by about 3% heading into the Democratic National Convention, and the most recent NPR/PBS News/Marist poll also found Harris up by 3%, suggesting a similar national environment could be waiting in November.

In recent elections, the framework has also hinted at the direction of public sentiment ahead of the general elections, like the 2022 midterms. Democrats finished with a 10.4% margin in Washington’s primary election, a down year that suggested Republicans were favored by about 1.6%, closer to the final national popular vote advantage of about 2.8% for the GOP in the House.

In 2020, a 14% primary margin suggested a 2% popular vote victory for Democrats, which ended up being closer to 3%. 2018’s “blue wave” that saw Democrats win the House popular vote with a roughly 8.5% margin was foreshadowed by Washington’s primary that saw a 20.4 % Democratic margin across its Congressional seats.

Why Washington state?

J. Miles Coleman, Associate Editor of Sabato’s Crystal Ball at University of Virginia Center for Politics, says Washington’s predictive power is driven by a few unique factors.

“One, the primary is open to all voters, and two, it occurs relatively late in the primary season,” he said. “Down-ballot primaries in other states usually begin in March, so it almost has a feel of being a ‘dress rehearsal’ for the fall election.”

Unlike many other primaries, Washington’s election sees all candidates from all parties on the same ballot, with the top two vote-getters proceeding to the general election. There are often more candidates and more ideological options for voters to express their preferences under the umbrella of the two major parties.

“This, combined with the very high turnout we see in this election – not quite at the level of a general election, but far better than most primaries – makes it much more representative,” Lakshya Jain, CEO of the elections modeling website Split Ticket, said. “It’s also a state that’s roughly as white as the nation is and just a few points more educated, meaning that it’s at least somewhat representative of the overall electorate.”

Jain said the Washington primary indicates an election cycle that is definitely driven by Democratic enthusiasm, but not to the degree of “blue wave” election years like 2018.

“The fact that Democrats are substantially outperforming their 2022 numbers bodes very well for them, especially given that they almost kept the House that year,” he added. “I will say that basically everything that’s been going on of late seems good for them — polling has surged in their direction, and the primary results and specials broadly all seem good for them.”

That said, the Washington primary shorthand should not be used as absolute gospel to be applied equally because there are 435 House races, 33 Senate races and 50 state-level presidential elections that all have their own quirks and local electorates that change how useful the estimate may be.

There are only a small handful of competitive House, Senate and presidential battlegrounds, so increased Democratic turnout and enthusiasm in noncompetitive races could lead to a higher national popular vote victory without winning those races, for example.

Coleman added that a third presidential election cycle that has been dominated by Trump means low-propensity voters that support him and no other candidate are another group to watch for as well.

“While there are some promising signs for Democrats, the Washington primary being one of them, to me, one X-factor lingering over the election is the question of how many of those voters Trump will bring out.”