That said, even in open primary states, primary election participation is low outside of presidential contests. I served as a poll worker once for a US House/Senate primary, and my precinct got about 7% turnout, and I'm in an open-primary same-day-registration state.
"Santos ran as a Republican for the United States House of Representatives in New York's 3rd congressional district, against Democratic incumbent Tom Suozzi, launching his campaign in November 2019.[24] Normally, the Nassau County Republican Committee, known for the tight control that its leadership exercises over often competitive races for its nominations, would have discouraged an unknown candidate with such minimal experience. However, Suozzi was expected to win the race easily, and no other candidates had put their names forward."
If you otherwise meet the requirements for the seat--citizen (usually), resident , not a felon--you too can find a ballot line for a contest a party expects to lose.
I should add that I have no idea whether Santos actually was on the primary ballots.
I don't have the data or time offhand, but if you're interested, have a look at ourworldindata.org; they probably have many relevant things.
The effect in California seems to have been to make the state even bluer, but arguably that's because rather than being forced to choose between Republican and Democratic standard bearers, the system returns some advantage to more moderate candidates, just within the field of Democrats (increasingly many of whom were once registered as Republicans). Unless you want to run on stereotypical Republican issues (abortion, religion, etc), your best bet strategically where the electorate is already majority Democratic is to run as a Democrat, regardless of your policies, which is not an option where candidates have to worry about being "primaried".
Ranked-choice is even better, of course, but I think you can see a similar dynamic wrt the candidate pool shifting Democratic.
The problem is our electoral system is a sham. Not just the mechanics (FPTP aka plurality voting is one of the worst systems) but the concept of "money=speech" which is really "money>speech". The 2 party system actively disenfranchises 30-50% of the electorate. The electoral college narrows the Presidential election to a half-dozen states. Gerrymandering warps whose votes actually matters. The list goes on and on. And most of these problems are present in primaries as well.
[1] https://theintercept.com/2024/05/03/portland-aipac-susheela-...