...almost happened.

The [California] legislature of 1859, which was intensely pro-slavery, passed a bill, which the Governor approved, to set off six southern counties and form a separate territorial government for them; the people of these counties themselves voted 2,477 for, 828 against dismemberment, and the results of the vote and the act were sent to the President and Congress. But the intense national excitement over the questions which led to the Civil War delayed action, and nothing ever came of this movement in the interests of the pro-slavery element in California.

- CHAB news, found thru links on No Pasaran