It’s not really clear, though, that a shirt that says something about a person who is on the ballot this year isn’t “campaign material,” because how are we defining (or does New Hampshire define" the time period for voting?
Certainly, the ads currently running on TV are considered “campaign ads” by most people, even though Election Day isn’t today.
The exact text of the NH law is:
No person shall distribute, wear, or post at a polling place any campaign material in the form of a poster, card, handbill, placard, picture, pin, sticker, circular, or article of clothing which is intended to influence the action of the voter within the building where the election is being held.
The clause “…intended to influence…where the election is being held” seems clear to me that it is referring to the election in which people are actively voting, not any election is the future.
That gets into some interesting areas, though. For example, Matt Mowers (who was on the ballot in that election, and won the Republican primary in New Hampshire’s 1st congressional district) ran on a campaign platform emphasizing his connections to and endorsement by Trump; I think one could make a decent argument that arguing for or against Trump is also arguing for or against Trump’s appointees and preferred candidates.
I think the Supreme Court’s 2018 ruling on Minnesota’s law covers that pretty well. That ruling blocked Minnesota’s law banning “political” apparel as too vague.
There is no problem with banning items supporting or opposing candidates or ballot measures, [Chief Justice John Roberts] indicated. But Minnesota also barred materials “designed to influence or impact voting,” which officials interpreted to cover messages touching on any subject that had been addressed by candidates and their parties.
“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,” Chief Justice Roberts wrote.
I think any argument that the NH law should be extended to not only candidates and measures on the ballot, but also any people or positions those candidates support, would be rejected for the same reason.