Recently, "#Calexit" has been trending in California, a type of secessionist movement similar to Britain's exit ("Brexit") from the European Union. By popular vote, Britain approved the move, which has no legal effect according to the High Court but which continues to affect the political sphere.
Could Californians undertake a similar move and vote to "exit" the United States? The short answer is, probably not.
It's worth emphasizing I only examine whether California could vote to leave the United States--but there is some question on the merits I'll mention below. I defer to international law experts about the legality of such a move, but there is some history suggesting it could not, at least on its own, do so.
First, the #Calexit movement is not terribly sophisticated. It's worth noting this effort started months ago and has since been adopted as the new vessel for secessionists. It is not clear whether the proponents intend a ballot initiative, referendum, or an advisory question. (Following some of their comments on social media, proponents use terms like these interchangeably, if not randomly.) The form matters, which I'll broadly outline below.
A ballot initiative would not be permitted. Ballot initiatives in California include proposing new statutes or constitutional amendments. It is not clear that either could properly authorize secession from the United States. A constitutional convention might, I suppose; but that does not occur via initiative.
A referendum also would not be permitted, because it is, well, impossible. In California, a referendum is a decision by the people to ratify or reject a law enacted by the state legislature. Because there is no secessionist law that the state legislature has enacted, there is nothing for a referendum to do.
Instead, an advisory question would be the means to recommend #Calexit, and it would be that--a recommendation by the people, something like a public opinion poll but carries greater weight having come from the ballot box. Even that is limited.
For starters, a citizen-led advisory question is not permitted in California. In American Federation of Labor v. Eu (1984), the California Supreme Court held that the people had the initiative and referendum powers, but those powers extended only to those matters that enacted laws. Advisory questions were not authorized. It explained:
We acknowledge the arguments of the proponents that there may be value to permitting the people by direct vote not only to adopt statutes, but also to adopt resolutions, declare policy, and make known their views upon matters of statewide, national, or even international concern. Such initiatives, while not having the force of law, could nevertheless guide the lawmakers in future decisions. Indeed it may well be that the declaration of broad statements of policy is a more suitable use for the initiative than the enactment of detailed and technical statutes. Under the terms of the California Constitution, however, the initiative does not serve those hortatory objectives; it functions instead as a reserved legislative power, a method of enacting statutory law. The present initiative does not conform to that model.
But a precedent has now been set in California on a different type of advisory question. The California legislature passed what would ultimately become Proposition 59 on the 2016 general election ballot. Some litigation kept a similar proposal off the 2014 ballot, but after full briefing the California Supreme Court considered whether the legislature could refer an advisory question to the people via initiative--here, a call to California's elected officials to lead an effort to amend the United States Constitution to permit greater regulation of campaign finance.
In Howard Jarvis Taxpayers Ass'n v. Padilla, the California Supreme Court permitted such an advisory question to appear on the ballot. The Court accepted the argument that the legislature had "the inherent power to conduct an investigation in order to select the wisest policy course." It could then refer such questions to the people as a part of its investigatory power. But the Court was careful to limit this power: among other things, "the investigative power permits inquiry only into those subjects 'in reference to which [the Legislature] has power to act.'"
An advisory question, then, could not ask for California to secede if the legislature lacked the power to secede. Instead, it could only ask, along the lines of Proposition 59, to urge elected officials to pursue (amicable?) secession with the federal government. That is, unless (and this is on the merits, as mentioned above) the state legislature does have some right to do so, but that is a much more complicated question--even though, I think, the answer is probably no.
That said, the (dare I call them unsophisticated) claims from the current #Calexit movement suggest they will be gathering signatures for this ballot measure, which suggests they do not intend to have the legislature refer the advisory question to the people. (It's also deeply unlikely that the state legislature, if asked, would do so.) Proponents apparently intend to simply gather signatures. And whether they do so as a ballot initiative (which the people lack to enact as a matter of law) or an advisory question (which the people lack the authority to do under the initiative power), the effort would likely fail, and the people could not vote for #Calexit.
UPDATE: The "Yes! California" movement helpfully directed me to their proposal they filed last year. It is an initiative that requires a regular referendum to call for secession, and in the event of a referendum that approves secession, triggers obligations from the California government to pursue good-faith secession negotiations with the federal government. I am fairly confident that laws that purport to bind future legislatures (if not all government officials) to act in a particular way run afoul of basic principles of legislative autonomy. (See generally People's Advocate v. Superior Court (Cal. App. 1986).) Finally, such a transformative change in California is likely a "revision" to the state Constitution, not simply an "amendment," and as such would need to come from the legislature and not an initiative petition.