The Supreme Court has rejected an appeal to reopen an investigation into whether Wisconsin Gov. Scott Walker's campaign to fend off his ouster from office illegally coordinated with outside conservative groups.

The justices on Monday left in place a Wisconsin Supreme Court decision that shut down the probe of the Republican governor. Walker won his recall election in 2012.

A group of prosecutors began an investigation the same year into whether Walker's campaign coordinated with Wisconsin Club for Growth and other conservative groups on advertising during the recall without reporting the groups' contributions.

Conservative-leaning justices who control the Wisconsin Supreme Court halted the investigation last year. They ruled that the coordination amounts to free speech and isn't subject to disclosure requirements.