Former Senate candidate Terrence Wall (R) accused the Republican expected to face Sen. Russ Feingold (D-Wis.) of using bribery and coercion to win the party's endorsement at the state convention last month.
A spokeswoman for upstart Senate candidate Ron Johnson (R) was quick to dismiss the allegations as "sour grapes."
"There is no evidence associated with Mr. Wall's claims," Kristin Ruesch, a spokeswoman for Johnson, said in an e-mail to The Ballot Box.
Johnson, a wealthy businessman, was the surprise winner of the endorsement at the May GOP convention in Milwaukee with 64 percent of the vote.
Wall told a reporter from WTDY's "Sly in the Morning" on Friday that "the process was corrupted."
"The campaign at the convention demonstrates the problems that we've got," he said. "We've got a handful of people who came in and spread money around."
Wall said supporters had come to him to say that their hotels rooms were paid because they voted for Johnson, while another man said he took votes for Wall and "[put] them in his pocket so they weren’t counted."
"I've got over 100 stories like that of corruption, bribery, you know, coercion," he said.
Johnson's campaign said it would provide hotel receipts to refute Wall's charge. "The claims are categorically and completely false," Ruesch said.
Johnson still faces businessman Dave Westlake in the September primary. Westlake, who has been critical of Johnson, said he didn't see anything wrong at the convention.
"Look, we all thought Wall was going to win," Westlake told the Wisconsin State Journal. "But I didn't see anything, or hear of anything, like (vote tampering or buying votes) going on."