In 1982, the U.S. Supreme Court ruled that the Socialist Workers Party is entitled to an exemption from reporting its campaign contributors, because the evidence showed that persons identified with the SWP were likely to be harassed. The Federal Election Commission has been exempting the SWP ever since. However, the FEC always sets a time limit at which the exemption expires. It is about to expire. The party has again asked for an extention. See this article in The Militant, the party’s weekly newspaper.
Other parties that have got similar exemptions have been the Communist, Freedom Socialist, and Socialist Alternate Parties. However, none of them has had any candidates for partisan office in many years.