Union shops are usually more expensive than non-union shops. Democrats on whose campaigns I've worked on have always stressed using union shops. Inevitably some one mentions that they are sure they can get the postcards printed at a local printer for a lot less. And, inevitably, the candidate will them that Democrats use union shops because Democrats support unions.
Back home in Kansas we also used union shops for local candidates. This became an issue a couple of times when Democrats were running in non-party races for the city commission. The pro-developer paper published an article excoriating as hypocrites our progressive candidates because they used a printer in a nearby town while their campaigns promised to support local businesses. The paper mentioned several local printers that were passed over. The response was simple, "they used a union shop".
In a later city office campaign cycle we got smear postcards. While some of the people worked on responding to it, several of us looked for who sent them. A month or so previous to the postcards were some polling calls. It got to be a local controversy. I posted about it at the time:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=153x1261All in all, it is easier to figure out who may be responsible for things like mailings when there's a union label. The union label has the "local" number and location. From there, you start checking possible campaigns' or PAC's spending in the city where the postcards originate from. Most anti-Democratic party morons use local shops (often, one owned by them or a friend) or mass-marketing shops. You can trace the origination of these things via the bulk mailing imprint.
A lot of times, these guys have businesses and they use their business bulk mail stamp to spew their hatred. More than a few times we'd also check out return P.O. Box number and match them up to local businesses. Those guys are easy to find (thanks to their stupidity).
The way we caught the guy in the story I linked to above was through the postcards. We used a local forum to put pressure on the local media. There were a lot of threads that talked about what was going on and a few of the forum participants (including me) started doing some snooping and reported what we found. We found out not only who the printer was but the guy behind it. Our grassroots reporting forced the local newspaper and television station to cover it.
Anyway, those mailers give people a lot of information that people can use to find out who is behind stuff. And, people working together on the ground to do some investigating can make a difference.
Keep an eye out for that union label.