"Twitter Expert" Tyson here. Two points worth noting regarding Twitter impersonation/name squatting when it comes to political campaigns:
First, Twitter's policy on "parody" accounts is fairly liberal. If it is clear that an account is a parody account, Twitter will likely let it slide. In trying to take down the account, the candidate or campaign will need to convince Twitter that the name squatter is actually confusing voters and/or the general public regarding the "tweeter's" (I still dislike that word) identity. The "impersonation parody policy" is available [here].
The second point is that a candidate, particularly a prominent candidate (or campaign) should (and used to) be able to get a "verified" account. This helped Twitter users
know whether the account was the "true" account associated with the candidate or campaign. http://twitter.com/#!/
In the mean time, those victimized by name squatters should use the lack of a verification system as an additional reason for taking down impersonation accounts. Point this out when filling out the take-down form. "This individual is impersonating me and I can't stop it. Moreover, I can't verify my actual account. I have no way of proving to the Twitterverse (another word I dislike) which account is the true account." Twitter doesn't want to be involved in fights. Especially big, potentially legal fights, with well-funded campaigns and public figures (and those who have legal rights associated with their publicity rights).
The take-away: If someone is trying to pass themselves off as you, you will be able to get the account taken down. If someone is making fun of you, the account will stay up. If it is tough to tell, Twitter will probably take the account down but there is no certainty. Check out the parody policy linked-to above and then argue that the account does not meet the criteria for a parody account.
Self promotion time: Check out my blog: Social Media, Esq.™
