Like I said, though, it’s really, really common - and by no means the exclusive province of groups on the left or the right. Often it takes the form of an interest group letter-writing campaign to lobby a Congresscritter or group of Congresscritters. The interest group essentially writes up several different letters urging the Congresscritter to support/oppose Bill X. These letters are then sent to the group’s local affiliate, who then asks its members to pick a letter, insert their name, and send it to the Congresscritter and/or local paper.

This can be more effective than the usual “postcard” campaign where the interest group sends out postcards (with postage prepaid) with preprinted letters urging passage/opposition to the bill in a mailing to its members. In that case, the members then only need to fill out their names and addresses and drop the card in the mail. The problem with this approach, with which a lot of people are familiar, is that it is transparently a form letter - as soon as it reaches the Congresscritter, there’s a good chance it gets thrown out without so much as getting logged.

The difference in this case is that you have the politician using this form of campaigning to lobby a group of people rather than a group of people using it to lobby a politician. But I doubt this is the first time a Presidential campaign has adopted the practice. FWIW, I don’t think it’s an unethical practice when an interest group does it; on the other hand, it certainly “feels” sleazier when a politician does it to get letters into the paper, even if I can’t rationally say why.