Liberals in general, really. It's the most efficient way to get as many voters to as many polling places as possible in the shortest frame of time. Watch the busses. See where they go, then see where they go next. Dollars to donuts, they don't stop at just one polling place, and dollars to donuts, the whole group will empty out of the bus and vote at each stop.
We would all be very, very grateful if some enterprising FReepers would follow a few of them, see if this happens, and video-document it. You have to shine a light on corruption.