I thought only the Rs had a WA primary today?
Correct. The Dems had a caucus.
Republican’s and Democrats have already had caucuses, but the people also have mail in ballots and today is the last day for them to be mailed in. On the ballot you have to mark if you are voting Democrat or Republican. So if you mark republican you have to choose one of the republican’s on the ballot. Same if you mark democrat you have to choose one of them.