What happens if someone is illegitimately removed from this database? How can you show whether it was a glitch, or deliberate? How do you know if the information they have about you is even right, or get it changed if you need to? Where's the accountability?

See the UK Post Office accounting scandal, in which a persistent computer error went unfixed for decades and caused hundreds of post office employees to be fired and dragged through courts for corruption that never happened. A good chunk of them committed suicide. The government and the software company both knew about the bug causing the issue, too, but prosecutions continued. "If the computer says it, it must be right", sort of danger.

Citizenship is already required to vote in state and federal elections.

Then you should have no problem with this because it won't stop anyone who is allowed to vote from voting, will it?

Everyone who votes absolutely should be checked to see if they are allowed to vote by citizenship. The only reason to disagree with this would be if you want non-citizens to vote in elections that they're not supposed to.

There was talk of establishing a national ID in the 90s but republicans revolted saying it would be the number of the beast. I guess their principles on the matter depend on whether the beast in question is Trump or Clinton. There would definitely be scope creep for such an ID, much like the SSN. I say just make it a national ID and eliminate the need to register to vote.