I would be outraged at this violation of my privacy
Public records....
Your voting history is not private, other than who you voted for. Anyone can look up when you voted, what your party is, etc.
It’s not a violation of anyone’s privacy. It uses public record data (who is registered to vote, who has cast ballots in past elections) which is available to anyone/any campaign that requests it. Data aggregators assemble this information and sell it to campaigns, but again, it is derived from public information sources.
The Cruz campaign does what other campaigns are doing, which is to merge this kind of data into a file on registered voters, and combine it with predictive modeling which scores each voter for propensity to support Cruz and propensity to actually get out and participate in the caucus process. The folks who score high on both attributes are the “given” support. The people that they are trying to motivate are the folks at least marginally inclined to support Cruz but not inclined to participate on Caucus day. That’s the segment that would receive this mailing.
Love it or hate it, it’s not a privacy violation and it is not prohibited by law or regulation. The first time I saw it was in 2012 when Obama For America used it. Other campaigns are using it now, and political parties have used generically in the past two presidential cycles.