But there are differences, too: Primary turnout is less volatile than in special elections, which often have ultralow voter participation. Primaries occur in almost every jurisdiction in the country, while special elections (particularly if analyzed without adjusting for population) are often deeply unrepresentative. And primaries are partisan ballots, not contests between two candidates of different parties, so you can interpret their results as a more direct proxy for partisanship preference.

State and District

The national primary vote might be somewhat predictive of the national House vote, but it isn’t nearly so simple to make a similar prediction by state or district.

How could that be? In most states, voters are confined to one party in the primary but may vote for whoever they like in the general. So the primary vote is an imperfect measure of partisan preference, not a measure of how voters feel about a particular race.

But that doesn’t mean it’s useless. Presidential vote choice, the most common indicator of a district’s partisanship, is also a deeply imperfect measure, so there’s value in adding the primaries to the mix.

There are places like West Virginia, where Republicans dominate in presidential elections but where the right kind of Democrat can take advantage of the state’s latent Democratic tradition. Conor Lamb might be doing so in the special election in Pennsylvania’s 18th District, which adjoins West Virginia and still has a Democratic registration advantage. And there are places like Orange County, Calif., or the suburbs of Dallas and Atlanta where one could wonder whether a strongly Republican tradition will cut against Democratic chances in places where Mr. Trump struggled.

The primary vote is another good measure of the partisan makeup of a state.

Take one of this year’s first primary states: Illinois. Based on recent presidential election results, it might appear that a Republican incumbent, Mike Bost, should be well positioned to hold a district that Mr. Trump won by 15 points. But this district has a strong Democratic tradition; Senator Tammy Duckworth actually carried Mr. Bost’s district by more than a 10-point margin against an incumbent Republican last November.