The majority’s ruling found that a reference to “political purposes” in the state campaign finance law was too broad unless applied only to advocacy for a particular candidate. That conclusion, the ruling said, essentially invalidates prosecutors’ theory of the case. “It is utterly clear that the special prosecutor has employed theories of law that do not exist in order to investigate citizens who were wholly innocent of any wrongdoing,” the ruling said.

In a dissenting opinion over the campaign finance element of the case, the court’s longtime chief justice, Shirley S. Abrahamson, considered part of the court’s liberal bloc, issued a stinging critique. “Lest the length, convoluted analysis and overblown rhetoric of the majority opinion obscure its effect, let me state clearly: The majority opinion adopts an unprecedented and faulty interpretation of Wisconsin’s campaign finance law and of the First Amendment.”

The climate on the court has become increasingly polarized and antagonistic as the split between the larger conservative bloc and its liberal one has been seen to widen. In April, Justice Abrahamson filed a federal lawsuit after voters approved a Republican-led constitutional amendment changing the way the chief justice is picked — in essence, assuring that a member of the conservative bloc, Patience Roggensack, would be chosen to replace her.

Legal experts said the special prosecutor in the recall case, Francis D. Schmitz, could appeal Thursday’s decision to the United States Supreme Court on several grounds, including a question over whether some of the judges should have recused themselves. Mr. Schmitz did not respond to an email on Thursday.

Court documents show that Mr. Schmitz had asked at least one justice to step aside from the case because of potential conflicts; legal experts say they suspect that the court’s four conservatives were asked to step aside because some of the conservative groups that were under investigation spent money that critics say was aimed at supporting the justices’ elections in previous years.