To understand the John Doe probes that have continued to dog Gov. Scott Walker, it's instructive to remember the "caucus scandal" from a decade ago. For many years Republican and Democratic legislators maintained caucus staffs that essentially were paid by the taxpayers to campaign on public time. In 2002 prosecutors launched an investigation of ringleaders in this sleazy system, which resulted in convictions of five legislators and several staff members from both parties.

The convictions reaffirmed what had always been the law: that public officials and their staffs must not conduct their campaigns on the taxpayer's dollar. That lesson was reaffirmed yet again by the John Doe probe launched by the Milwaukee District Attorney's office in its investigation of Walker's staff during his time as Milwaukee County Executive.

The probe convicted six people of crimes, including three staff members of Walker, all charged with doing campaign work on government time.

Tim Russell, former deputy chief of staff to Walker, pleaded guilty to stealing from a veterans fund, but in a sentencing memo, assistant DA Bruce Landgraf said Russell did considerable campaign work on county time. Russell had regular phone contact with campaign operatives, helped publish a pro-Walker campaign blog and helped conduct opposition political research for Walker's bid for governor.

Kelly Rindfleisch, another Walker aide, was sentenced to six months in jail for campaign fundraising at the courthouse and using a secret email system located 25 feet from Walker's office. The system allowed staff to evade open records requests and leave no trail of their campaigning on county time. Rindfleisch was quite experienced in such chicanery: She had been threatened with prosecution and granted immunity in the caucus scandal.

Darlene Wink, Walker's constituent services coordinator, pleaded guilty to two misdemeanors for doing campaign work while on the county clock.

All of which suggests Walker tolerated staff who illegally campaigned on government time. Though Walker insisted he was not a target of the investigation, he hired two top-level criminal defense lawyers to represent him and paid them nearly $200,000 from a defense fund he created.

So one could imagine investigators continuing to look for evidence that Walker was breaking campaign laws. And last February, just one month before the first John Doe probe came to an end, a second investigation began. Langraf appointed longtime federal prosecutor Francis Schmitz to lead the new probe and retired Appeals Court Judge Gregory Peterson to oversee it.

Precisely what prosecutors are investigating is not known, but a preemptive attack on the probe -- published by The Wall Street Journal -- provides some clues. The paper published an opinion piece assailing the investigation as "government enforcement power" used to "stifle free speech." Meanwhile, the article revealed that Schmitz "has hit dozens of conservative groups with subpoenas demanding documents related to the 2011 and 2012 campaigns to recall Gov. Walker and state legislative leaders." The subpoenas demand "all memoranda, email... correspondence, and communications” related to the campaigns from such groups as the League of American Voters, Wisconsin Family Action, Wisconsin Manufacturers & Commerce, Americans for Prosperity-Wisconsin, American Crossroads, the Republican Governors' Association, Friends of Scott Walker and the Republican Party of Wisconsin.

One target of the subpoena is Eric O'Keefe, director of the Wisconsin Club for Growth. He told the newspaper that "at least three of the targets had their homes raided at dawn, with law-enforcement officers turning over belongings to seize computers and files."

The new probe appears to be looking at potential coordination -- which would be illegal -- between the Walker campaign and third party groups. Just as Walker had an overlap between his county staff and gubernatorial campaign, in the recall election he had an overlap between his campaign and an independent third party group: R.J. Johnson was an adviser to both Walker's campaign and the Wisconsin Club for Growth.

Conservatives like Milwaukee radio host Charlie Sykes have declared the investigation a "witch hunt." Critics carp that the investigation appears to be looking only at conservative groups and not labor unions or others who backed Democrats in the recall elections. Why aren't they going after Walker's opponent in the recall election, Milwaukee Mayor Tom Barrett, some ask.

But the second Walker probe appears to have grown out of the first, which found criminal wrongdoing by his staff. There have been no such accusations against Barrett. And Schmitz, the prosecutor in the second probe, was one of three lawyers on President George W. Bush's short list for U.S. Attorney in 2001, which hardly suggests a liberal bias.

One veteran Republican quoted by Milwaukee Journal Sentinel columnist Dan Bice called the first John Doe probe a "fishing expedition" that "produced almost nothing in the way of substantial criminal behavior." Members of both parties made similar complaints about the caucus investigation. But I think the average citizen is glad that the lawbreakers involved in the caucus scandal and in Walker's county exec office were brought to justice. Clean government and clean campaigns make for a stronger democracy.

Bruce Murphy is the editor of UrbanMilwaukee.com.