By SCOTT BAUER, Associated Press

MADISON, Wis. (AP) — Democrat Russ Feingold, one of the most outspoken critics of political spending by corporations and outside groups, challenged Republican Sen. Ron Johnson on Friday to agree to keep third-party money out of their U.S. Senate race in Wisconsin.

Johnson's campaign had previously asked Feingold whether he would disavow money from political action groups, and it's been one of the larger unanswered questions since Feingold announced plans on May 14 to seek a rematch against Johnson. Johnson's spokeswoman Betsy Ankney said she hadn't had time to review the pledge and had no immediate comment.

Johnson defeated Feingold, who was seeking a fourth term in the Senate, in 2010. Feingold stopped the Democratic Senatorial Campaign Committee from spending money on television ads to support him in that race, and he refused to back the creation of a super PAC to help his campaign. A tally by the Sunlight Foundation showed independent groups spent relatively little in that race, about $3 million, mostly on the Republican side.

But outside groups were expected to weigh heavy in the rematch because Democrats are targeting Johnson as they try to regain the Senate.

Feingold's proposal is nearly identical to the pledge that Democrat Elizabeth Warren and Republican Scott Brown agreed to in Massachusetts' U.S. Senate race in 2012. The so-called Badger Pledge is designed to prevent any third-party group from airing television, radio or online ads considered to be an independent expenditure or issue advocacy for the duration of the race.

Should a group spend money helping either campaign, the beneficiary would have to pay half the cost of the ad to a charity of the opposing candidate's choice. The pledge also requires both candidates not to coordinate with any third-party group on issue advocacy ads.

Feingold said in a statement that the pledge is the only way to "limit the tens of millions of dollars from outside Super PACs and dark money groups on both sides of the aisle."

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