One of the most basic principles of constitutional law is that the Constitution only binds the government -- not private individuals -- so you can't accuse fellow citizens of violating your constitutional rights just because their actions discourage you from exercising your own constitutional rights (like someone kicking you out of a dinner party at their home because they don't like your viewpoint, which makes you more reluctant to express your views in the future). This is called the "state action" doctrine.
But this basic principle of Constitutional Law 101 seems to have eluded groups like the United Steelworkers Union and Common Cause, which claim that Clear Channel billboards in Ohio and Wisconsin that declare that "Voter Fraud Is A Felony" are "voter intimidation" that violates the “right to vote” and federal voting-rights laws. (Under pressure from these groups, Clear Channel took down the billboards, which had been paid for by an anonymous foundation.)
Even assuming that these billboards (which are factually correct statements of the law) were somehow objectionable, and that they somehow discouraged people from voting, they would still be protected political speech. A billboard that discourages you from voting is protected speech unless it contains "true threats." And no federal "voting rights" laws can reach such protected speech without both violating the First Amendment, and exceeding Congress's enumerated powers. The Fifteenth Amendment, which protects voting rights against state action, does not change this. Section 2 of the Fifteenth Amendment, authorizing Congress to pass legislation protecting voting rights, does not authorize Congress to regulate private conduct, as opposed to governmental conduct. (See James v. Bowman (1903), involving the 15th Amendment, and United States v. Morrison (2000), involving similar enforcement provisions in Section 5 of the Fourteenth Amendment.)
Even speech that pressures or emotionally intimidates people is protected speech if it does not threaten violence. (See NAACP v. Claiborne Hardware Co. (1982) (political boycotting designed to pressure people was protected). Calling speech "discriminatory" (which the billboards aren't) does not change this. See White v. Lee (2000) (Fair Housing Act could not constitutionally be applied to public speech opposing minority housing project, since such speech was protected even if it incited illegal discrimination). Labeling otherwise protected speech "intimidation" does not strip it of First Amendment protection. See also Bauer v. Sampson (2001) (invalidating discipline under "workplace violence" policy for ridicule of college president that depicted imaginary violence against him); UWM Post, Inc. v. Board of Regents of University of Wisconsin System (1991) (voiding ban on speech that created "intimidating" environment).
(Speaking of Common Cause, which claims that the billboards are "intimidation": it helped organize a “protest action,” at which demonstrators vilified two private citizens, calling for them to be "quarantined" based on their political donations, and called for the lynching of Clarence Thomas. Maybe it should stop facilitating "intimidation," rather than falsely accusing others of intimidation.)