The holiday differential is likely enough that the employer will not allow the employee to work any longer than is strictly necessary, and only if the workplace absolutely must stay open during a holiday. Most people will not even have the option to work. For those that do, the shifts will be offered to the most senior workers first.

Some states might even allow absentee ballots for those who will be present in the state, but are essential personnel who have been scheduled to work on election day. That's not something that can be established federally, though.