He's breaking like five FEC and FCC rules that prohibit a candidate from actively promoting himself on TV on his own show. It's the same reason that they had to stop showing Schwarzenegger movies when he was running for governor of California.
All someone has to do is complain, and Colbert is ruined...
Any person may file a complaint if he or she believes a violation of the Federal Election Campaign Laws or Commission regulations has occurred or is about to occur. The complaint must be made in writing, and sent to the Office of General Counsel, Federal Election Commission, 999 E Street, N.W., Washington, D.C. 20463. The original must be submitted along with three copies, if possible. Facsimile transmissions are not acceptable.
A complaint must comply with certain requirements. It must:
* Provide the full name and address of the person filing the complaint (called the complainant); and
* Be signed, sworn to and notarized. This means that the notary public's certificate must say "...signed and sworn to before me..."
Furthermore, in order for a complaint to be considered complete and proper, it should:
* Clearly recite the facts that show specific violations under the Commission's jurisdiction (citations to the law and regulations are not necessary);
* Clearly identify each person, committee or group that is alleged to have committed a violation (called the respondent);
* Include any documentation supporting the allegations, if available; and
* Differentiate between statements based on the complainant's (the person who files the complaint) personal knowledge and those based on information and belief. Statements not based on personal knowledge should identify the source of the information.
http://www.fec.gov/pages/brochures/complain.shtml#anchor460355
http://www.slate.com/id/2176466/nav/tap3/