The Ted Cruz campaign has been especially aggressive in Iowa with a mailer that highlights the poor record of some voters of participating in caucuses.
The thing came in a brown envelope (above) and consisted of single sheet of yellow paper (below) marked like a ballot that showed a score and percentage grade for voters named on the sheet.
These folks don’t typically turn out to vote, so Cruz was trying to goad them into participating. Voting records are public in some American states so campaigns can tell who voted and who didn't. Your neighbours.
You can tell if it worked by the results from Monday’s caucuses.