Cal 3
Cal 3 (also called Proposition 9) is a halted 2018 voter initiative to split California into three states. It was introduced by Tim Draper. If it passed, California would not be immediately split. The U.S. Congress would need to approve.
Supporters of the split said the state is too large to govern efficiently, and those against said the split is a waste of time and resources.
The initiative was taken off of the ballot in July 2018 by the Supreme Court of California. Tim Draper called the decision "corrupt."
Analysis
California is currently a stronghold for the Democratic Party. If the state were to split in three, Southern California could become a swing state. Splitting the state would also create 4 more Senate seats. The Republicans would not want all those 4 extra seats to go to the Democrats.
All three potential states voted for the Democratic nominee in the 2016 presidential election.
Polls
References
2018 California elections