I can tell you that campaigns have much of this information already, often provided directly by the state as public information. The idea that campaigns don't already "create Custom Audiences based on voting history and district" is laughable at best. Communications are often targeted in exactly this way.
"Voter registration information is public record in Florida with a few exceptions. Information such as your social security number, driver’s license number, and the source of your voter registration application cannot be released or disclosed to the public under any circumstances. Your signature can be viewed, but not copied. Other information such as your name, address, date of birth, party affiliation, and when you voted is public information."
Here's Florida's relevant information:
http://dos.myflorida.com/elections/for-voters/voter-registra...
"Voter registration information is public record in Florida with a few exceptions. Information such as your social security number, driver’s license number, and the source of your voter registration application cannot be released or disclosed to the public under any circumstances. Your signature can be viewed, but not copied. Other information such as your name, address, date of birth, party affiliation, and when you voted is public information."