Not just in theory.
Article II, Section 1
The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice-President, chosen for the same Term, be elected as follows:
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole number of Senators and Representatives to which the State may be entitled in the Congress: ...
Not just in theory.
The article states that California's measure stands a slim chance of becoming reality because it "could go into effect only if states with a combined total of 270 electoral votes — the number now required to win the presidency — agree to the same process."
Doesn't that limit California's ability to determine how their electoral votes are cast?