Can a US citizen who has never lived in the US, register for voting in a midterm election? If so, can this citizen pick any state for voting registration? Do any states make this process particularly easy if this process does exist?
Can a US citizen who has never lived in the US, register for voting in a midterm election?
Maybe. Most states will allow such a person to vote in a federal election if the person had a parent whose last domicile in the US was in that state, or who was last registered to vote in that state. See the Federal Voting Assistance Program (FVAP) page on voters who have never resided in the US for more information. Of the first three states, alphabetically, Alabama is absent from that page, and Alaska and Arizona have different requirements:
A U.S. citizen who has never resided in the U.S. and has a parent or legal guardian that was last domiciled in Alaska is eligible to vote as a federal voter and may vote for federal offices only.
A U.S. citizen who has never resided in the U.S. and has a parent or legal guardian that was last registered to vote in Arizona is eligible to vote in Arizona.
Colorado extends the requirements to include other family relationships:
A U.S. citizen who has never resided in the U.S. and has a parent, legal guardian, spouse or civil union partner that was last domiciled in Colorado is eligible to vote in Colorado.
If so, can this citizen pick any state for voting registration?
No. Since the requirements are based on the residence or voter registration of various family members, one could perhaps be able choose among several states. It's also possible that there would be no state in which such a person would be eligible to register.
Do any states make this process particularly easy if this process does exist?
I am not aware of any difference from one state to another. The application process is more or less standardized through the Federal Post Card Application. The deadline for registration varies by state, as do the deadlines for requesting an absentee ballot and submitting an absentee ballot. Details may be found in FVAP's guide.
First, your reference in all these matters is https://www.votefromabroad.org/
You cannot vote in any old state. If you have lived in the US, by federal law you must use the last address at which you were registered to vote. Or the last address you were officially resident at, if you left before you were of age to register or you never did. If you are the child of a US citizen and never lived in the US yourself, you must use the same information for your parent(s), last address of registration or failing that last address of residency.
This can be a bit involved as you can imagine.. And much depends on the goodwill of the registrar in the county/state in which you are technically required to register, and their willingness to accept your documentation. But it CAN be done - while I was in France with Democrats Abroad, we successfully helped register people who were children of Americans who'd been resident since WWII, and had died since, and one whose last address had been razed and replaced by a freeway to boot :-). Note I'm not saying that it can be done without a lot of tedious transcontinental paper gathering.
The bad news is that you're likely starting this process way too late. You should at this time have your ballot in hand and if possible have returned it already to ensure its timely arrival and proper counting. Since a handwritten signature is necessary, paper mail is your only option. Most states have already closed any easy online registration process, even California. But it's worth trying, or at least starting the process so that you may be able to vote in the next elections. Much will depend on whether you're trying to register in California, New York or .. Alabama, North Dakota? But don't despair, help is available and we have a LOT of experience in this. Please start now!