Six states are currently considering some form of birther-inspired legislation. Sounds like harmless loonery, right? Check out the language in the Arizona bill:
The secretary of state shall review the affidavit and other documents submitted by the national political party committee and, if the secretary of state has reasonable cause to believe that the candidate does not meet the citizenship, age and residency requirements prescribed by law, the secretary of state shall not place that candidate's name on the ballot.
As David Weigel points out, this gives the secretary of state a lot of authority. Mere "reasonable cause" to suspect that a candidate is not a citizen is enough to get that candidate taken off the ballot. And remember, plenty of Republicans say something along the lines of, I'm not saying Obama isn't a citizen, I'm just saying I'd like to see the evidence so we can put this to rest.