[JeffAtlanta]

It does not matter how many states pass such a law, to the extent that it affects the power of non-cooperating states in the selection of the President, it is an unconstitutional confederation (Article I, section 10).

Couldn't California and other states just put in their state law that it won't take effect until a certain number of other states do as well?

This wouldn't be a contract, compact or confederation as agreements with other states would be not be necessary.

[Right Wing Professor]

Couldn't California and other states just put in their state law that it won't take effect until a certain number of other states do as well?

Case law says reciprocal legislation is a compact. See the link I posted.