There's relatively little regulation of campaign speech.  Broadcast law has long required stations to make airtime available to candidates for public office and give all of them equal opportunities.  That usually means that stations have to be willing to sell advertising time, not give any for free.  Occasionally libel suits arise from heated exchanges between candidates.  The Federal Election Commission imposes some limits on candidate spending, which indirectly affects candidates' efforts to communicate their messages.  The Supreme Court has struck down some limitations on campaign spending as a violation of First Amendment speech rights. 

The Federal Election Commission's web site has a host of information about federal regulation of campaigns; some of the rules affect campaign speech.  The Commission answers FAQ about campaign activity and the Internet. 

Buckley v. Valeo, 424 U.S. 1 (1976) (leading Supreme Court decision that raises First Amendment questions about efforts to limit contributions to, and spending by, candidates for office). 

State v. Jade, 554 N.W. 2d 750 (Minn. Ct. App. 1996) (striking down law prohibiting false campaign speech).

State ex. Rel. Pub. Disclosure Comm'n v. 119 Vote No! Comm., 957 P.2d 691 (Wash. 1998) (striking down law prohibiting false campaign speech).

Randall v. Sorrell, 126 S. Ct. 2479 (2006) (striking down Vermont's limits on candidates' campaign spending as violating the First Amendment's protections of free speech).

Lee Goldman, "False Campaign Advertising and the 'Actual Malice' Standard," 82 Tulane Law Review 889 (2008).

Allison R. Hayward, "Regulation of Blog Campaign Advocacy on the Internet: Comparing U.S., German, and EU Approaches," 16 Cardozo Journal of International & Comparative Law 379 (2008).

Jacqueline D. Lipton, "Who Owns 'Hillary.com'? Political Speech and the First Amendment in Cyberspace," 49 Boston College Law Review 55 (2008).

Benjamin Norris, "Fired Up! In the Blogosphere: Internet Communications Regulation Under Federal Campaign Finance Law," 84 Washington University Law Review 993 (2006).

David Stevenson, "A Presumption Against Regulation: Why Political Blogs Should be (Mostly) Left Alone," 13 Boston University Journal of Science and Technology Law 74 (2007).

D. Wes Sullenger, "Silencing the Blogosphere: A First Amendment Caution to Legislators Considering Using Blogs to Communicate Directly with Constituents," 13 Richmond Journal of Law and Technology 15 (2007).