Among Republicans, Tommy G. Thompson raised $833,912 - a figure that puts him at the top of fund-raising in the four-way primary.
Electronic filings were not available on Monday, but the candidates either provided some details of their contributions or summaries of their campaign statements filed with the Federal Election Commission.
The deadline for filing statements was Sunday.
In the Republican race, Thompson was followed by former U.S. Rep. Mark Neumann (R-Wis.), who raised $733,450, according to his campaign statement.
Meanwhile, the campaign of Eric Hovde said that the Madison business executive raised $237,211, and over the past four months he has raised nearly $350,000.
The campaign of Assembly Speaker Jeff Fitzgerald (R-Horicon), who has been lagging in the polls, said Fitzgerald raised $41,033 and $144,000 total for the campaign.
What remains unclear until the candidates' statements are posted online is the amount of money that Hovde, the wealthiest candidate in the Senate race, is personally handing over to his campaign.
The Thompson campaign said Thompson provided his campaign $7,500 in the second quarter and $32,500 so far this year.
Neumann, who spent millions of dollars on his gubernatorial bid against Scott Walker in 2010, gave no money in the second quarter, his campaign statement shows.
Neumann has benefited from spending of nearly $700,000 by the Club for Growth. The anti-tax group has run ads that have attacked Thompson and Hovde. The Club for Growth says that it spent $429,440 this month on advertising and related costs to attack Hovde and $268,415 against the former Wisconsin governor.
In a statement, the Hovde campaign said that 70% of his donations in the second quarter were less than $100 each, and that 80% of contributions were from Wisconsin donors.
The Hovde campaign declined to detail how much he's spent to bankroll a multimillion-dollar ad campaign that's helped to raise his profile in the run-up to the Aug. 14 primary.
The Journal Sentinel reported last week that political sources in both the Republican and Democratic parties said Hovde had spent about $4 million on radio and TV time since April.
Hovde disclosed in a financial statement filed with the U.S. Senate this month that he had made loans to his campaign between $1 million and $5 million. Based on his financial statement, the Journal Sentinel estimated that Hovde's assets from real estate and financial investments could range as high as $240 million.
"I would expect Hovde to spend whatever resources he needs to, to win," said University of Wisconsin-La Crosse political scientist Joe Heim.
Big spending ahead
The Wisconsin race could help decide control of the Senate, which is now in Democratic hands. That means a huge infusion of independent and out-of-state funding between now and November, Heim predicted.
Hovde has surged with the help of his ad campaign, which has turned a little-known candidate into Thompson's leading challenger, according to polls.
Thompson led Hovde 35% to 23% among likely Republican primary voters in a Marquette University Law School poll released Wednesday. Neumann followed at 10% and then Fitzgerald at 6%, with 25% undecided.
A separate poll released on July 10 by Public Policy Polling indicated a much tighter race. The poll showed Hovde leading Thompson, 31% to 29%, with Neumann at 15%, Fitzgerald at 9% and 16% undecided.
Both polls were conducted between July 5 and 8.
Heim said Thompson's lead in fund-raising among Republicans, with Neumann close behind, means that "moneyed forces in the Republican Party think that Tommy is going to win," while Neumann is likely tapping into pockets of conservatives and voters with the strong religious convictions.
Still, he viewed the Republican fund-raising as lackluster.
"It could be the recall of Governor Walker tapped out a lot of traditional Republic sources," Heim said.
Baldwin's tally shows that Democrats know who their candidate will be and don't need to sit back to write campaign checks.
"It also suggests that Tammy Baldwin knows how to raise money," she said.