The 1st Amendment to the Constitution guarantees the right of peaceful assembly which is the justification for political parties to exclude non-members. However, because of the size of the major parties their primary elections are typically funded by taxpayers. To exclude independents from primaries can be considered a violation of the 14th Amendment. If a state referendum allows independents to vote in partisan primaries the Supreme typically concurs.

In some states major political parties avoid the issue by conducting semi-open primary elections. Residents usually don't declare a party preference when registering to vote. Any registered voter can vote in either - but not both - major party primary.

Some states conduct fully open non-partisan primary elections. California conducts a top 2 primary with only two candidates for an office advancing to the general election. This guarantees a majority vote winner but decreases candidate diversity in the general election.

I prefer semi-open primaries with ranked choice voting general elections. Candidates can qualify for the general election by winning a primary, by nominating convention of a state recognized political party, or by submitting petitions with a specified number of signatures. This results in candidate diversity in the general election and encourages voter participation.