I'm a Vietnam Vet. To me a Kerry sticker, displayed on my property where the public can see it is egregious.
I'll give you a for instance. One of my son's used to have a Seattle Mariner's bumper sticker on his car. I asked him to remove it or not park in our drive way.
Why? Because the Seattle Mariners is the only major league baseball team who refuses to fly an American flag anywhere on the outside of the building or where it can be observed on the outside of the building. They had one displayed for two days then removed it because of public complaints (this is Seattle).
If the Mariner's cannot display out flag I will not allow anything supporting them to be displayed on my property. When I explained to this my son he removed the sticker and he and his wife sold their season tickets (he did send a letter to the Mariner's management first and was blown of).
My property...it's pretty simply.
No political clothing is something where he would be on much firmer ground but it would extend to the parking lot and the cars parked there. Most places have a "no alcohol on company grounds" policy even when the bottles are sealed and locked in your trunk so there certainly is president for this.