English to Spanish Other Government / Politics elections

The Washington State Supreme Court has upheld the blanket primary as protecting compelling state interests "allowing each voter to keep party identification, if any, secret; allowing the broadest possible participation in the primary election; and giving each voter a free choice among all candidates in the primary."

No estoy segura si *blanket primary* son las elecciones primarias "generales" porque también existe la expresión general elections.
¿Alguien sabe bien a qué se refiere ese **blanket**? Muchas gracias!