As the ongoing tally in the Washinton State Democratic Primary highlights, early voting by mail has thrown a wrench into primary voting. Bernie Sanders and Joe Biden, the two remaining candidates, are splitting about 75% of the vote. 25% of the vote has been largely disenfranchised by voting for a candidate that subsequently dropped out of the race.
The same thing happened here in California.
In 2016 the majority of Republicans did not support Donald Trump in the primary. He got a consistent 30+%, yet he ended up the with the nomination because there were so many other candidates more moderate voices could not sort themselves out to support a single candidate, so Trump won enough primarys to lock up the nomination.
The current primary system on both the Republican and Democratic side favors ideologically pure candidates at the far ends of the political spectrum. In large states where folks can vote by mail people vote days or even weeks before election day, often before candidates start dropping out. The ideologically pure candidates have their following but more moderate candidates tend to be splitting votes with a number of other joderate candidates.
The Democratic and Republican party should use ranked choice voting in primaries. That will make it more likely everyones vote will count even if some candidates drops out, and it probably would produce a more cooperative and less partisan politics.