The 7th U.S. Circuit Court of Appeals on Thursday rejected the Government Accountability Board request.
The board asked to intervene so it could make its case about what campaign activity can be regulated. Prosecutors worked with the board in launching the probe into whether Walker's campaign and others broke state laws.
The Wisconsin Club for Growth sued in federal court and a judge in May stopped the probe. Prosecutors have appealed to the 7th Circuit.
While the board's request to join the lawsuit was denied, the appeals court says it can file its own separate legal brief.