The Supreme Court holds that a Minnesota law that prohibits people from wearing political clothing in a polling place violates the First Amendment.
The case is Minnesota Voters Alliance v. Mansky, decided on June 14. This issue is more complicated than you think. At first glance, what's wrong with wearing a Hillary for President t-shirt at the polling place? Would that unduly influence anyone that day? But remember that many states, including New York, prohibit campaigning within 100 feet of any polling place, and those restrictions are legal.
The Court starts with a forum analysis. The legality of speech restrictions on government property is determined by first examining what kind of forum we are talking about. A public forum, such as a public park or street corner, is a speech haven so long as you don't kill anyone or block traffic. At the other end of the spectrum is the nonpublic forum, places where the property was not traditionally intended to promote speech and whose particular purpose would be undermined by unfettered speech. Like a military base. Or, according to the majority in this case, the polling places in Minnesota, where on Election Day they are "government-controlled property set aside for the sole purpose of voting," subject to greater speech restrictions.
But that does not end the analysis. True, the nonpublic forum holding gives the plaintiffs an 0-2 count, with one strike to go. But the Court comes around to the speech side in the rest of its opinion, holding that while the state is allowed to restrict some forms of advocacy at the polling place, the restriction against "political" clothing and badges is too vague to survive the First Amendment. What does "political" mean? "It can encompass anything of and relating to government, a government, or the conduct of government affairs," the Chief Justice writes for a 7-2 majority. So that even a button that implores people to vote would fall under this prohibition.
While the Court seems to be OK with restrictions against clothing promoting a political party or the name of a candidate, the restrictions also prohibit "issue-orientated material," which can include anything and everything that might conceivably have a political connotation. "A rule whose fair enforcement requires an election judge to maintain a mental index of the platforms and positions of every candidate and party on the ballot is not reasonable. Candidates for statewide and federal office and major political parties can be expected to take positions on a wide array of subjects of local and national import." The Court similarly rejects the rule against "promoting a group with recognizable political views," defined as "the issues confronting voters in a given election," which, again, can include anything. So that ACLU or Chamber of Commerce apparel -- to extent anything like that actually exists -- would be prohibited in polling places as well.