> California could make this change by referendum.
No, it could not, because Article I, Section 3 (emphasis added): "The times, places and manner of holding elections for Senators and Representatives, shall be prescribed in each state by the legislature thereof; but the Congress may at any time by law make or alter such regulations, except as to the places of choosing Senators." (the last part of that about choosing Senators has its effect eliminated by the 17th Amendment, but that isn't important here.)
And Congress has exercised its authority in U.S. Code Title 2, Section 2c (emphasis added): "In each State entitled in the Ninety-first Congress or in any subsequent Congress thereafter to more than one Representative under an apportionment made pursuant to the provisions of section 2a(a) of this title, there shall be established by law a number of districts equal to the number of Representatives to which such State is so entitled, and Representatives shall be elected only from districts so established, no district to elect more than one Representative (except that a State which is entitled to more than one Representative and which has in all previous elections elected its Representatives at Large may elect its Representatives at Large to the Ninety-first Congress)."
> why would they if other states retain a system that disproportionally skews sits towards one party?
Because your constituents are better represented. California strikes me as a potent place to do this because I could see a constitutional amendment passing at the ballot box.
But they aren't better represented unless everyone else does the same.
Suppose California were to do it, resulting in a proportional allocation of seats in the House for its delegation. If this causes the House to swing from Democratic majority to Republican majority, the net effect is the opposite of what most Californians wanted.
Don't get me wrong, I get the point that it is a more fair and equitable way of doing things, and in principle, I agree. But if you play fair at a table where everybody else cheats, you lose. My state (WA) also has referendums, and if such a proposal would come up, I would absolutely vote against it - unless it was some kind of interstate compact where another similarly-sized red state were to implement the same reform at the same time.
I’ve had this exact thought: that Texas and California should have some sort of compact to do it at the same time. That would be a boon for Texas Democrats (of whom there are many) and California Republicans (ditto).
No, it could not, because Article I, Section 3 (emphasis added): "The times, places and manner of holding elections for Senators and Representatives, shall be prescribed in each state by the legislature thereof; but the Congress may at any time by law make or alter such regulations, except as to the places of choosing Senators." (the last part of that about choosing Senators has its effect eliminated by the 17th Amendment, but that isn't important here.)
And Congress has exercised its authority in U.S. Code Title 2, Section 2c (emphasis added): "In each State entitled in the Ninety-first Congress or in any subsequent Congress thereafter to more than one Representative under an apportionment made pursuant to the provisions of section 2a(a) of this title, there shall be established by law a number of districts equal to the number of Representatives to which such State is so entitled, and Representatives shall be elected only from districts so established, no district to elect more than one Representative (except that a State which is entitled to more than one Representative and which has in all previous elections elected its Representatives at Large may elect its Representatives at Large to the Ninety-first Congress)."