Last Thursday, Frank Knaack of the ACLU posted this diary about wearing campaign clothing, buttons, etc. to the polls. His bottom line is worth repeating:
Unless you know for sure, we suggest that you either refrain from wearing political clothing, or be prepared to cover up or remove such clothing. Though you may be permitted to vote with it on, you also risk losing your right to vote.
The rules vary from state to state. I was curious as to just how many states do ban campaign apparel. I was surprised at the number that do; I only found five states (edit: now six, counting Iowa) that explicitly allow it. See below the fold for a state-by-state list. If the state name is hyperlinked, it goes to an official statement from a state government website. Unofficial sources (including news accounts) are listed after the state name.
The following states do restrict wearing campaign-related clothing etc. at the polling place. If you live in one of these states, leave the gear at home, or be prepared to take it off and/or cover it up.
Alaska (no official source found, anectodal evidence here)
Arizona (unofficial source)
Arkansas (unofficial source here)
California
Colorado (according to this unofficial source, I am unable to verify)
Connecticut
Delaware
District of Columbia
Georgia
Illinois (unofficial source)
Kansas
Maine (unofficial source)
Massachusetts
Michigan
Minnesota
Mississippi
Missouri (unofficial source)
Montana
Nebraska
New Jersey
New Mexico (anecdotal source)
New York
Ohio (unofficial source)
Oklahoma (unofficial source)
South Carolina
South Dakota (unofficial source)
Tennessee (Knox County website, couldn't find it on the state website)
Texas
Vermont
Virginia (unofficial source here)
West Virginia (unofficial source)
Wisconsin (voters are "asked not to wear campaign paraphernalia")
The following states explicitly permit wearing campaign-related clothing etc.:
Alabama
Florida
Kentucky (unofficial source here, but note this comment: "Kentucky Board of Elections attorney Katie Gabhart [...] warned that local authorities could try to press charges if someone were dressed like 'a walking placard.'"
Iowa
Maryland
North Carolina (unofficial source)
In the following states campaign apparel may or may not be allowed depending on where in the state you are:
Nevada (unofficial source) will ask you to remove anything easily removed like a button. They say nobody will be turned away for wearing a shirt, but you will be asked to leave the polling place immediately after voting.
New Hampshire allows local discretion:
We recommend that Moderators establish and uniformly enforce a policy regarding what can be worn or displayed inside the polling place. If you have a policy that requires voters to remove buttons or cover clothing that contain electioneering messages, we suggest you post a sign to that effect near the entrance door to the building.
Pennsylvania is also going on a county by county basis (see DKos diary)
Wyoming (bars electioneering but leaves the definition of that up to individual county clerks - unofficial source)
I have not found specific information for the following states. If anyone knows specifics, please comment. If you're voting in one of these states - don't take a chance.
Hawaii (they have a factsheet on campaign signs but it does not explicitly mention apparel)
Idaho
Indiana
Louisiana
North Dakota
Rhode Island
Utah
Washington (see UncleDavid's comments downthread which suggests they are allowed)
Finally, I understand that you can wear anything you want to vote in Oregon, the only state that votes entirely by mail.