EPA Administrator Gina McCarthy will announce the proposal on Monday. Getty Images WASHINGTON—The Environmental Protection Agency will propose mandating power plants cut U.S. carbon-dioxide emissions 30% by 2030 from levels of 25 years earlier, according to people briefed on the rule, an ambitious target that marks the first-ever attempt at limiting such pollution. The rule-making proposal, to be unveiled Monday, sets in motion the main piece of President Barack Obama ‘s climate-change agenda and is designed to give states and power companies flexibility in reaching the target. But it also will face political resistance and become fodder in midterm congressional races, particularly in energy-producing states, and is destined to trigger lawsuits from states and industry that oppose it. The rule would affect hundreds of fossil-fuel power plants—hitting the nation’s roughly 600 coal-fired plants the hardest. The carbon framework seeks to strike a balance between what environmentalists want—an ambitious overall […]