On February 16, it was revealed that California Congressmember Barbara Lee has filed with the Federal Election Commission to run for U.S. Senate from California in 2024. Already two other California Democrats had declared for that seat, Adam Schiff and Katie Porter. A possible fourth Democratic contender is Congressmember Ro Khanna.
If four Democratic congressmembers run in the California top-two primary, that opens the possibility that none of the Democrats would place first or second, if there are two Republicans who run and who both have substantial support. In 2012, in the 31st U.S. House district, a district which was predominently Democratic, two Republicans and four Democrats ran, and the two Republicans placed first and second because the four Democrats split up the Democratic vote.
If two Republicans ended up on the November 2024 California ballot for U.S. Senate, and with no write-in space on that ballot, disenfranchsing Democrats, that would be the end of the top-two system in California. Democrats would go all-out to repeal it.