I didn't think voters were allowed to have political clothing either? I've heard of some voters asked to turn masks and shirts inside out while they're at the polls.
The PDF linked from the post clarifies the rule for PA: "Wearing a t-shirt or button supporting a candidate, campaign, or political party (except voters in the act of voting)"
(Other jurisdictions may have other rules, of course)
(Other jurisdictions may have other rules, of course)