On Capitol Hill, House Democrats have weakened a landmark greenhouse gas emissions bill in what they call a necessary move to win broader support. The measure would cap US emissions at a certain level and allow polluters to buy pollution credits that would ostensibly cancel out their emissions. On Tuesday, bill sponsor and House Energy Committee chair Henry Waxman said the bill’s emissions cap has been reduced from 20 to 17 percent, and its required percentage for drawing electricity from renewable sources dropped from 25 to 15 percent. Waxman said the reductions were necessary to win the support of Democrats backed by coal and other major industries.