It's worse than that. If "Southern California" is a swing state, that means the best we do is that the Democrats get four more Senators, and we get two more Senators, so that's a break even from the status quo. So the "upside" is a break even.
But if that swing state goes the other way, that means that instead of current California having two Dem Senators, the three state version gets six. No way. That's a huge downside.
That is what I am tryng to say
California now has 2 dem senators.
Split into 3, it would have 2 in "Cal" and 2 in "NorCal", and "SoCal"
may have 2 GOP senators, so that would be a wash.
each party gains 2 that didn't previously exist.
However, "SoCal"
could elect 1 each or 2 dems.
Best case scenario is break even, worse is to lose the 1 or 2 we would be counting on in "SoCal", going over to the dems.
It is really not a swing of 6, but only 4 max (still not good) as the Dems aren't likely to lose the 2 they have now, from whatever region, if nothing changes at all.