Of all the indicators, primary elections are probably the single messiest measure of the national political environment. From state to state and cycle to cycle, voters may either have a very compelling reason to show up — or no reason to vote whatsoever. A strong or weak Democratic or Republican primary turnout can mean absolutely nothing.

But if all the states are added together, the vagaries of individual state primary elections more or less cancel out. Over the last few decades, partisan primary turnout does correlate relatively well with the results of midterm elections.

In 15 primaries since the court’s ruling, 52.5 percent of primary voters have cast Republican primary ballots compared with 48 percent in the same states in 2018, according to data compiled by the pollster John Couvillon. The last midterm is used as the point of comparison because of the one-party presidential primary in 2020.

Of course, 2018 was a good year for Democrats. In the end, they won 54 percent of the major party vote and carried the House easily. So they have room to fare quite a bit worse than they did in 2018 and still put up a respectable showing. Indeed, a 4.5-point shift from 2018 would yield a pretty close House national vote, with maybe a slight Republican edge depending on how one looks at uncontested races.