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...