One of President Bush’s most ambitious environmental proposals is in jeopardy — the goal of cutting mercury emissions from coal-burning power plants 46 percent by 2010. Many in the utility industry are complaining that such a requirement, which is part of Bush’s “Clear Skies” legislation, would cost far more than expected and could force some coal-fired plants to switch to cleaner-burning natural gas. Republicans from the Midwest, home to many of the nation’s dirtiest coal-burning power plants, are insisting that the mercury provision in the bill be rewritten — and the administration seems content to go with the flow rather than defend its initial proposal. “They touted it as a big initiative, and now they are quietly tiptoeing away from it,” said Frank O’Donnell of the Clean Air Trust. Mercury, which most often makes its way into humans through consumption of contaminated fish, can cause birth defects, lung damage, behavioral changes, and neurological damage to young children, among other health problems.