On October 21, the California Secretary of State issued this press release, confirming that the Republican, Green, and Peace & Freedom Parties will not let independents vote in their 2020 primaries. The same three parties made the same decision in 2016.
The California Republican Party did let independents vote in its 2004 presidential primary, but in that primary, President George W. Bush was the only name on the Republican presidential primary ballot.
How can registered NPP voters request a Democratic ballot absentee?
When they request their postal ballot, they can request a Democratic ballot.