Former Republican Governor of Wisconsin Tommy Thompson had expressed an interest in challenging Feingold, but ruled himself out of the race in April 2010. Johnson, a millionaire manufacturer and Tea Party movement favorite running for political office for the first time, was the frontrunner. In response to controversy over his ownership of stock in British Petroleum, Johnson said he would sell it when market conditions were favorable and possibly use the proceeds to help finance his Senate campaign. Johnson was endorsed by ; the Club for Growth, a fiscally conservative advocacy organization; former Wisconsin Republican Lt. Governor Margaret Farrow; Republican U.S. Senator Jim DeMint of South Carolina; and Wisconsin Republican U.S. Representatives Jim Sensenbrenner, Tom Petri and Paul Ryan. Businessman David Westlake was endorsed by the Republican Liberty Caucus. According to the Center for Responsive Politics, as of August 25, 2010, Johnson had invested $4.3 million of his own money into his campaign; this amount represented 71 percent of his campaign funds, while 27 percent of his campaign funds came from individual contributions.
Feingold's first television ad was a positive ad released in March. In July 2010, Feingold's second 2010 television election ad attacked Johnson for alleged support for offshore drilling in the Great Lakes. Johnson quickly countered Feingold with a television ad of his own. Feingold's logo was Moving Forward. In one ad, he emphasized independence and called himself a "penny pincher." Johnson argued that manufacturers and accountants were underrepresented in the U.S. Senate, and there were too many lawyers. Feingold received the endorsements of the Milwaukee Journal Sentinel and the Green Bay Press-Gazette. The Beloit Daily News endorsed Johnson.
