lol last time I voted I was wearing this shirt
This depends STATE BY STATE (or town by town), so check out your own state’s rules. As for everywhere, only poll workers are banned from this, and people holding up signs for candidates have to be a certain distance from the building.
It is to keep disagreements from flaring up.
At the polls I work at, we ask people to cover or remove political clothing.
We also have a large wind breaker for folks to wear while voting.