The granddaddy of guidelines is The Communications Act of 1934 which outlines how radio stations are expected to treat political candidates when it comes to programming and purchased advertising. Knowing some of the highlights may explain a few questions you might have had.
Hi, The Head Moose Sent Me
The law describes what a legally qualified candidate actually is - in case some wacko does walks into a radio station wearing a moose head and demands to get "the political rate" to run ads for his newly established "Antler Party." Section 73.1940 goes through a list of criteria a radio station can look at if there is any question. Although too long and wordy to get into here, candidates have to publicly announce their intention, must legally be qualified to hold the office they seek, (for instance: the age requirement for President is 35), and a few other things. In short, the law gives the radio station ammunition - should it need it - to refuse nut cases and anyone else who does not deserve to be treated as a bona fide candidate.
Equal Time or No Time
From a programming point of view, the law, Section 73.1941, also instructs radio stations to treat candidates equally. If a station is going to feature the head of the Antler Party in a one-hour discussion about spending more public funds to save the elusive Winky Worm, it must extend a timely invitation to the Beaver Pelt Party, haters of the Winky Worm who feel it's an environmental nuisance. Conversely, if the station wants nothing to do with keeping track of equal time, then it can refuse to let ALL the candidates come on-the-air and run their mouths about the Winky Worm. It's all or nothing and once this Pandora's Box is opened, there's no going back. If the station let's the Antler Party have air time, then the other legitimate parties get equal time like Beaver Pelt Party, the Shaky Leg Party, and the Granite Gringo Party.
There's another legal kicker about appearances on-air: when radio stations decide to give time to candidates, the station is not permitted to censor anything the candidate says. Most stations are not comfortable with this because it is potentially problematic.
Cheap Radio Ad Thrills
The law has a built-in perk for all qualified candidates who want to run advertisements on radio stations. 45 days preceding a primary or 60 days out from a general or special election, if a candidate wants to buy 100 30 second commercials, the radio station must charge the candidate "the lowest unit charge of the station for the same class and amount of time for the same period."
In other words: if the candidate wants to run commercials just between 6 - 10 a.m., the rate must be equal to the lowest rate any other client is getting during that period. Since radio stations often discount rates based on how much a client buys, the political candidate benefits from a big client who has purchased 1000 commercials at a substantial discount. They both pay the same rate, even though the candidate might be running 7 commercials. Section 73.1942 says "A candidate shall be charged no more per unit than the station charges its most favored commercial advertisers for the same classes and amounts of time for the same periods."
Bathroom Reading for Politicos
The law instructs radio stations to keep a political file for public inspection which keeps track of requests for broadcast time from candidates, the status of those requests, if ads were purchased, when the commercials aired, what rate was charged, and what class of time was purchased. Any free time provided to candidates must be recorded as well. And the station has to keep this paperwork on hand for at least two years before it can be boxed up and sent to the U-Storage locker down the street or used as kindling.
Real Life Repercussions
The Communication Act of 1934 says "licensee shall have no power of censorship over the material broadcast by any such candidate." Most stations interpret that to apply to equal time afforded a candidate during programming OR paid advertising.
At times, there are candidates who meet all the criteria the law requires who walk into a radio station with a script or recording which is outright profane or offensive. There's little stations can do about offensive speech. The law protects the candidate. But, since the law also requires the radio broadcaster to limit profane speech to after 10 p.m. and before 6 a.m., the station might insist the ads run during that time period. A crazy candidate could find a crazier lawyer to pressure the station into a more general schedule but as long as the station does not censor the content and affords the candidate the correct rate, the station can stay legal. The station might even choose to run a disclaimer before an offensive or profane political ad airs.
I have given you an overview of the most important aspects of broadcast law when it comes to running political advertisements on radio. For more details, follow the links in this article. Overall, the law provides generous protections for political candidates for one very good reason: politicians wrote it.
(Disclaimer: I am not an attorney and this is not legal advice. This is my interpretation of the law as a layman who works in radio and has read the law. For legal advice, see a qualified attorney.)