Have agents sit outside polling places with facial recognition technology that flags anyone on that database that is likely to vote Democrat and detain a number of them until the polling ends then release them without charge.

I learned something interesting about the upcoming Brazilian elections the other day that seems intended to counter exactly this kind of thing: there's a law (and perhaps it's in the constitution, I don't recall exactly) that says that no one shall be arrested for a couple of days around the election date, unless someone is caught in the act of a crime or if it's for a major crime.