I think the better legal theories would be a tortious interference with contractual relations or interference with prospective business advantage. Easier to get around the public figure issues associated with a defamation/slander theory.
They were interfering simply because of their hatred for Limbaugh and a desire to see him fail in any endeavor he undertakes. Since Limbaugh said he wants to see Obama's policies fail, Obama's minions want to see Limbaugh fail in his desires, too.
-PJ