Nearly six months after a majority of British voters voted to secede from the E.U., interest was building among Californians on the possibility of a “Calexit” from the U.S. In fact, supporters were proposing a referendum to take place in 2019. Although the two exits would be comparable—two large states of empire-scale unions (California’s economy being larger than that of France, and California’s population being larger than that of Poland)—the reasons for a Brexit are more fundamental than those for a Calexit. As a result, the secession of Britain from the E.U. would have a firmer foundation in terms of political theory.
A fundamental disagreement on how much governmental sovereignty the E. U. should have played a major role in the popular interest in Britain to secede from the E.U. The difference can even be characterized as between confederalism, wherein the states retain sovereignty as in the U.S. Articles of Confederation (1781-1789), and modern federalism, in which sovereignty is “dual”—meaning that both the federal and state governments have some autonomous authority in basic law. In cases such as the E.U. and U.S., which are “empire-level” both in that their states were sovereign and in being so large that cultural differences exist from state to state, the difficulty in being unified is such that the states really need to be “on the same page” concerning the type of federal system. If the UK was an outlier among the E.U. states in viewing the E.U. as essentially confederal in nature, both the state and Union would be in a better position after a secession.
In the case of California, the issue was that 60% of the voters had voted for Hillary Clinton for U.S. President, while her opponent, Donald Trump, won the election. It was not that Californians suddenly believed that the U.S. system of governance (e.g., modern federalism) was itself problematic. It cannot even be said that California was alone in having a majority of the voters opposed to Donald Trump. Were a federal union to depend on the majority of voters in every state being for the winner of a major federal office, dissolution would be a natural result.
So I submit that in 2016 whereas the British vote to secede had a solid foundation in terms of federal theory, the Californians in favor of secession did not. At the empire-level, inter-state political differences can be expected in a union, whether it is federal or consolidated. Such differences are major if they incur the old saying, “a divided house cannot stand.” Such is the case in a Union wherein state governments hold different positions on whether the federal system is confederal or modern (i.e.,, whether governmental sovereignty stays with the states). Such is not the case, however, in a Union where some states, through their electors, vote for one candidate for federal president while other states vote for another candidate.
 Eugene Scott, “Interest in #Calexit Growing after Donald Trump Victory,” CNN, November 10, 2016.