Senator Challenges Rival to 'Super PAC' Truce Senator Jon Tester, Democrat of Montana and one of the most endangered incumbents up for re-election this year, became the latest lawmaker to challenge his competitor to agree to a truce in the war over third-party expenditures flooding Senate contests. Mr. Tester on Wednesday proposed that he and his Republican rival, Representative Denny Rehberg, call on all outside groups to halt television and radio advertising. If one side's allies violate the pact, that candidate would have to donate the value of the advertisement to charity, Mr. Tester proposed. "Let's do our part to steer this campaign toward Montana's values of honesty, accountability and transparency," he wrote in a letter to the congressman, who represents the entire state. Mr. Tester, in his first term, is following the successful efforts of Senator Scott P. Brown, Republican of Massachusetts, and his Democratic challenger, Elizabeth Warren, who have their own delicate pact to keep "super PACs" out of their race. Mr. Tester may have a tough time winning over Mr. Rehberg. As of Dec. 31, Mr. Tester had $3.8 million in his war chest, nearly double what Mr. Rehberg had. But Mr. Rehberg appears to have third-party allies lined up and willing to spend on his behalf. American Crossroads, the Republican behemoth with Karl Rove on its board, is already going after Mr. Tester, and fresh disclosures indicate it has millions more to spend on Republican candidates in the fall. "I don't see it as a desperation move at all," Mr. Tester said of his push for a truce in a conference call with reporters. I see it as the right thing to do. Mr. Rehberg's campaign manager, Erik Iverson, called the senator's proposal "interesting." "We are going to give it a close look," he said, "and we will respond in due course."