It would be wrong to put on a Biden t-shirt and walk into every democrat polling place you can find- but secretly vote for Trump.
Very very wrong.
Wrong and illegal. A shirt or button for any given candidate would count as campaigning within 100’ of a polling place.
A solid blue shirt would probably be OK, though...
Funny you should say that. I am in a deep blue district. I always pick up a democrat sample ballot and then give the Republicans a wink as I go in.
An honest election judge would insist that you turn the shirt inside out before being allowed inside.