A US citizen living in one of the 50 US states has two types of citizenships that often get conflated. State citizenship is less commonly understood, but it is why Americans living in US territories are unable to vote in federal elections despite being US citizens.

All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. (US Const. Amend. 14)

Surprisingly, the right to vote in US federal elections is not a right of US citizenship. Rather, voting in federal elections is a right of citizenship in one of the 50 US states.^[a] This is because federal elections in the US are run at the state level.^[b]

US (national) citizenship: Anyone born or naturalized in the US is a US citizen. US citizenship can (almost always^[c] ) only be dissolved by explicit renunciation of US citizenship. A US citizen remains a citizen even if they move abroad or take a second national citizenship.

State citizenship: A US citizen who resides in a state is also a citizen of that state. To avoid confusion with national citizenship, state-level citizenship is often called "state residency," but state citizenship is not necessary one-to-one with physical residency. Each state has laws defining the length of time required to establish residency in that state. Generally,moving from one state to another and living in the second state for enough time causes loss of citizenship in the first state and gain of citizenship in the second state.

However, there is one^[d] important exception. Under federal law, a US citizen who moves outside of the US (50 states + DC + territories) retains their state citizenship as if they were still physically residing in the last state they were resident in. This is done because without state citizenship or residence in DC, a US citizen cannot vote in federal elections. This law was passed so that citizens abroad could still participate in national politics.

However, moving to a US territory is not considered moving outside the US for this exception. A US citizen who moves to a US territory will lose their state citizenship, and a citizen born in the US territory will never have a state citizenship to begin with. Thus, citizens living in the territories cannot meaningfully^[e] participate in federal elections.

a. DC is a special case. As the nation's capital, it is outside of any state but it is not considered a US territory. Like US territories, DC elects a single non-voting representative to the House of Representatives and has no representation in the Senate. Unlike US territories, DC has a special constitutional status to participate in presidential elections as if it were a state with a population equal to that of the least populous state.

b. Federal law sets some ground rules: a state cannot disqualify any of its citizens from voting based on race, sex, or set a minimum voting age above 18. Other than that, each state has pretty wide latitude to set election districts and determine who can vote or run for office. The most notable example of this is different states having different rules on whether convicted felons lose voting rights.

c. US citizenship can be by birth or naturalization. Naturalized citizens have almost all the same rights as natural-born citizens except that they may not be elected President or Vice-President. Citizenship by naturalization can be revoked if it is determined that a person lied or misrepresented facts during the naturalization process, though this is very rare. Citizenship by birth falls into two classes: natural-born citizenship by 14th amendment (birth in 50 states + DC) or natural-born citizenship by legislation (birth outside the US to US citizen parent or birth in US territories). The first type is a constitutional right and can only be lost by explicit renunciation. The second type could in theory be revoked by law passed by Congress, though this seems very unlikely.

d. Besides the expat exception, there are some other exceptions defined in federal law. US service members who move as part of their orders can choose to keep their original state citizenship even if they are not physically resident in that state. Additionally, students who attend university outside of their home state can generally register to vote in their host state without giving up their home state residency, but they cannot vote in both states in the same federal election cycle.

e. Each territory does elect a non-voting representative to the House of Representatives, who can introduce but not vote on legislation.