The Federal Communications Commission (FCC) has a rule that says that TV stations must keep a list of political ad buys and make it available on request by the public. However, until recently, stations weren't required to post this data on the Internet, and so the only way to get the records was to physically travel to the station in person.

However, thanks to ProPublica's Free the Files project -- and especially their Free the Files volunteers!! -- this critical issue has been spotlighted and this summer the FCC passed a rule requiring the stations in the nation's top markets to upload the files to the FCC's website https://stations.fcc.gov/.

The system is far from perfect and has a lot of limitations -- eg. there's not a great search! -- but it's a good start.

Rachel Maddow highlighted this issue of transparency in political advertising on a recent show: