Perhaps the DNC bought the mailing list of all those whose paperwork is in progress.
I though it was illegal for USCIS to sell or disseminate that information.
USCIS doesn't have it but the Dept of Labor does. It's all a part of public disclosure. Anyone issued a green card through employment has all their personal information (address, phone number, employer, etc.,) listed in a downloadable database. It's part of a disclosure system to help make sure permanent immigrants aren't taking jobs away from U.S. citizens. But my information is more recent than the date ranges they have available.