PROVIDENCE, R.I. -- No Democratic lawmaker is being bombarded by more negative ads than Sen. Sherrod Brown (D-Ohio). So far, GOP-aligned groups have poured more than $7.5 million into TV ads to attack Brown, while progressive groups have spent just $1.5 million to help him.

"It has a corrosive effect," Brown said Saturday in an interview with The Huffington Post at Netroots Nation, the annual gathering of progressive activists and bloggers that was held in Providence this year. "It's just a drumbeat of one issue after another after another after another."

Brown is locked in a tight race against Ohio Treasurer Josh Mandel, and Republicans have made defeating the progressive senator a top priority. So far, only the League of Conservation Voters, Service Employees International Union and Majority PAC have donated money to help Brown.

When asked why he thought progressives weren't matching the conservative spending in his race, he said that first of all, they simply don't have as much money. But perhaps more fundamentally, conservatives have a greater incentive to donate to these groups in the first place.

"If George Soros or some wealthy person on the left drops $20 million into a race or a series of races, and his side wins -- our side wins -- they don't get any material benefit from it," he explained. "But if the Koch brothers or Sheldon Adelson or Harold Simmons in Texas put $10 or $20 million each in and win, they get tax cuts, weaker environmental laws and anti-labor legislation. They have more billionaires with more income, more money anyway, but they also have an economic incentive. For their side, it's a good investment to make more money. For our side, it's a good investment in good government and progressive values. And the financial incentive trumps so often."

He added that at this time, labor unions simply have "too many fights" to fight.

Michael Podhorzer, the AFL-CIO's political director, told The Huffington Post in April that the union had plans to ramp up its involvement in Ohio -- both because of the critical role the state will play in the presidential race and the desire to see Brown win a second term.

"There is nobody that is up this year that we are going to work harder to make sure is reelected. He has been as much a champion of working people as anyone. That's not lost or dismissed by labor leaders," he said. "We are going to put in whatever resources are necessary to win and it's our strategic judgement that what we are doing now is built around winning that campaign. We know we can't afford to lose Sherrod Brown in the Senate."

Brown dismissed the idea of entering into a contract with Mandel that would bar the involvement of super PACs in the race, saying he simply didn't believe the Republican side would keep its end of the bargain.

"I wouldn't trust them to keep it. Why would they keep it?" he asked. "Anybody who's willing to run racist ads or just say, 'I don't care what Politifact or Factcheck.org say about the truth. I'm still going to do it -- everything's a conspiracy in the liberal media.' Anybody that's willing to do and say all of those things is not going to honor any kind of contract like that."

In Massachusetts, Sen. Scott Brown (R-Mass.) and Democratic challenger Elizabeth Warren have such an agreement, where a candidate is penalized financially if an outside group jumps in on his or her behalf.

Brown also weighed in on the Wisconsin recalls, saying that while Republican Gov. Scott Walker's victory certainly wasn't a win for the labor movement, to characterize it as a "big defeat" was also an overstatement.

In November, Ohio labor unions and Democrats successfully overturned the state's anti-collective bargaining law, championed by Gov. John Kasich (R). The difference, however, was that in Ohio, the ballot referendum was about that particular law. In Wisconsin, it was about the governor.

Brown also noted that Wisconsin's recall election took place long after when the February 2011 protests against Walker first started, creating a long, divisive process for voters.

"It was tiresome and frustrating for large numbers of voters, and I think they just wanted it to end. It lasted a year and a half," he said. "And lastly, the voters in the end probably thought you don't remove a governor unless he's done some criminal activity, and there are accusations of that. I don't know if they're going to be true when they're finally dispensed with or not. So that was the difference between Ohio and Wisconsin. Then they had the money. Scott Walker could run a 12-month campaign and Tom Barrett had six weeks."

Also on HuffPost: