But Walker, a potential 2016 presidential candidate who is up for re-election in the fall, ignores the many Republicans involved in the probe, including the special prosecutor who says he voted for the governor.
No one has been charged as part of the investigation, known as a John Doe, which began in 2012.
Walker, who was in Eau Claire on Monday, argued the investigation was politically motivated as he made campaign stops around the state over the weekend. In the face of the accusations, the attorney for special prosecutor Francis Schmitz issued a statement Friday in defense of his actions.
"Mr. Schmitz swore an oath when he accepted his appointment to lead the John Doe investigations in accordance with the law and on behalf of the State of Wisconsin," said Schmitz's attorney Randall Crocker. "He has kept that oath. Mr. Schmitz, along with the attorneys he works with, are career prosecutors who make their decisions based on the facts and the law."
Schmitz is a former assistant U.S. attorney and finalist for the U.S. attorney position in Milwaukee when Republican George W. Bush was president. He was hired after five district attorneys - including two Republicans - said they wanted to avoid partisan concerns.
Walker doesn't talk about Schmitz but instead points to the fact that the investigation was launched by a Democratic district attorney in Milwaukee. He also points out that two judges - one state and one federal - rejected arguments made by prosecutors and put the investigation on hold pending appeals.
The fact that the prosecutor who started it is a Democrat, and he chose only to go after Walker and Republicans and no Democrats, legitimately raises concerns that the probe is politically motivated, said Rick Esenberg, an attorney and president of the conservative Wisconsin Institute for Law and Liberty.
"The notion that you would have a partisan district attorney embark on a multi-year investigation of a partisan of the other side and not expect there to be huge political blowback was, at best, naive," Esenberg said Monday. "I can hypothesize a number of reasons why someone would do this. Partisanship is one, but it's not the only one."
Characterizing the investigation as a partisan witch hunt "is just not based in truth or fact," said Jay Heck, executive director of the government watchdog group Common Cause of Wisconsin.
"It's very selective and it really ill-serves citizens," Heck said.
The allegations against Walker were made public Thursday by a federal appeals court judge who agreed to unseal records after no objections were raised by prosecutors or the Wisconsin Club for Growth, one of the conservative groups and allies of Walker being investigated.
Schmitz said in the December court filing that Walker was involved with a "criminal scheme" to illegally raise money and coordinate campaign spending among Walker's campaign and independent outside groups.
Walker doesn't talk about Schmitz when attacking the probe. Instead, he focuses on the fact that the investigation was started by Democratic Milwaukee County District Attorney John Chisholm and that two judges have since put it on hold.
"These are the truths that need to be stated over and over again to fight the slander directed at me and our campaign by my political opponents," Walker said last week.
One of the rulings stopping the investigation was made by U.S. District Judge Rudolph Randa. He sided with Wisconsin Club for Growth in May in finding that the investigation was a violation of the conservative group's free speech rights.
In a separate ruling last week declining to release even more records in the case, Randa said prosecutors were appearing to "seek refuge in the court of public opinion, having lost in this court on the law."
Randa, 73, was appointed to the federal bench in 1992 by Republican President George H.W. Bush. Randa is also an adviser to the Milwaukee chapter of the conservative Federalist Society.
The second ruling halting the investigation came in January from the judge overseeing the investigation, former state Appeals Court Judge Gregory A. Peterson. A reserve judge from Eau Claire County, it is not known whether he is a Republican or Democrat.
Both orders have been appealed by prosecutors.