Maybe mention a few of these things in your message. For more background, check out this article.
– Primary elections are paid for public money, not political parties. They should not be closed or exclusive.
– Oftentimes the most impactful way a person can vote is in a primary that is not their typical political party’s. But that doesn’t mean that a voter should not have a voice in who gets elected.
– Elections should be as open and accessible to as many people as possible. Putting a bridge between when a person has to register with one party interferes with that access.
– Many people do not identify with one major party or the other. They vote in particular primaries because they want to see the person elected who can best address the issues they care about.