“How do they know the party of early voters?”
Most often from voter lists from the primaries. Even in open primaries they have a list of who voted and in which party primary.
If you chose a Republican ballot in the primary, the odds are pretty good that you will vote that way in the general election.
A less accurate method is the area the voter is located. Some weighting math is then used to extrapolate how many tend to vote for each party in that area based on historical voting records.
>>Even in open primaries they have a list of who voted and in which party primary.<<
Not in my state. You are given ballots for both parties and vote one and toss the other. No record is made of how you voted, and registration is not party-affiliated. You’re just registered to vote.
I realize other states handle it differently, but that’s why we don’t see similar reports of early voting here.