That investigation exposed the insensitive email exchanges among his aides, like this chain email forwarded by a former chief of staff in the county office: “I can handle being a black, disabled, one-armed, drug-addicted, Jewish homosexual on a pacemaker who is H.I.V.-positive, bald, orphaned, unemployed, lives in a slum, and has a Mexican boyfriend, but please, Oh dear God, please don’t tell me I’m a Democrat!”

Mr. Walker has distanced himself from the conduct and denied knowledge of the campaign work being done, and he was not charged. But there were more disclosures.

In December, a campaign official for Mr. Walker was fired after a Milwaukee Journal Sentinel columnist reported that the official had tweeted several years earlier about a bus ride she took: “This bus is my worst [expletive] nightmare Nobody speaks English & these ppl dont know how 2 control their kids #only3morehours #illegalaliens.”

But if anything has defined Mr. Walker’s political life over the past decade, it was his drive to limit unions just six weeks into his tenure as governor in 2011, a move foretold in his years as Milwaukee County executive. Although Mr. Walker had made it clear as a candidate for governor that unions could expect to see change, his critics say he never made clear the extent of his intentions until he was elected.

But as county executive, he also clashed with public-sector unions, calling for 35-hour workweeks instead of 40, with corresponding reductions in compensation. He pushed to privatize cleaning and food-service workers. He demanded spending cuts and battled openly with the Board of Supervisors in a county that leaned Democratic. At one point, he went so far as to suggest that the county government itself might be abolished as a way to spare waste.

“It came from eight years of being a county executive,” he said in a recent interview in this southern Wisconsin village. “Nobody needed to tell me what needed to be done. Anytime I had a reasonable option, I’d get shot down by the public-employee union leaders who would rather lay off hundreds of people before they would take even a 35-hour workweek. So I had just grown so frustrated with them throughout the process that I said, ‘Something’s got to change.’ ”