Wisconsin Democratic Party Chair Mike Tate responded by saying that Walker is the first governor to ever need a criminal defense fund.

"He has bent and broken the rules that should govern the conduct of an executive," Tate said in a statement. "Every citizen, regardless of party, agrees that our government should be free of corruption. It looks very much like Scott Walker has not given Wisconsin this honor."

A February poll by Marquette University Law School found that Walker's support had dropped, and that the John Doe investigation may be a key factor. Walker has a 46 percent favorable rating, compared to 50 percent in January, according to the poll. Fifty-two percent of respondents aware of the probe said it was "really something serious," while 40 percent said it was "just more politics."