As serious as a foreign takeover! California becomes, by turns, Taxifornia, then Mexifornia, eventually, Aztlan. There are already politicans in various jurisdictions (mostly in SoCal, but a few in SF) openly calling for, not only illegal alien voting, but illegal aliens as elected officials. (Of course they don’t mean Irishmen, or Russians.) They see how the Mexican government gets to rip its citizens off, well beyond the ambitions of most US pols, and they want their shot at it.
If secession comes, Mexifornia will be the first, starting a chain of others.