Opposition by farm groups to the climate bill passed in the House of Representatives, which would mandate a U.S. cap on greenhouse gases, may impede the progress of the bill in the Senate-- where senators must represent the diverse needs of their state and have the power to filibuster on behalf of their constituents' interests.  "Agriculture can in effect hold this bill hostage. This suggests we're only at the beginning of the negotiating process," said SNRE and Ford School of Public Policy professor Barry Rabe in the article.