This has produced a clear geographic fault line that the EPA regulations should deepen. In both presidential and congressional elections, Democrats now rely mostly on states where their diverse and urbanized coalition dominates the population; that includes the East and West coasts, plus Sun Belt states fitting that model, such as Colorado and Virginia. Republicans now lean primarily on the South, as well as on heartland states that are older, more religious, more blue collar, and less racially diverse. (The biggest exceptions to this pattern are the burly Midwestern battlegrounds, such as Michigan and Ohio, where economic populism allows Democrats still to compete.)

Patterns of energy use closely track these political lines. Many red states are heavily invested in the fossil-fuel economy, either as producers of oil, natural gas, and coal, or as large consumers of low-cost, coal-powered electricity (partly because several are manufacturing centers). The blue states, with only a few exceptions, produce little fossil fuel, rely less on coal for electricity, and generate less carbon (partly because many have moved further toward a postindustrial, white-collar economy).

The result is that red states that are hostile to government activism also face greater material risks from the EPA regulations than blue states. Interest fortifies ideology. All 10 of the states that emit the most carbon per megawatt-hour of electricity generated voted for Mitt Romney in 2012; so did 14 of the top 20. By contrast, 15 of the 20 states that produced the least carbon per megawatt-hour backed Obama. Likewise, in four-fifths of the states Romney carried, per-person carbon emissions from all sources exceed the national average, federal figures show. Four-fifths of the states that backed Obama emit less per person than the national average.

In its proposed regulation, EPA attempted to accommodate these differences. The proposal, which maximizes state flexibility, generally imposes smaller percentage reductions through 2030 on the high-emitting states than it does on the low-emitting states.

None of that prevented red-state Republicans like Indiana Governor Mike Pence from denouncing the proposal. But blue-state leaders generally welcomed the rules. "They are very positive," says Mary Nichols, chairwoman of the California Air Resources Board. "There is a net benefit here to the West and to California from having stepped out early."

The regulations seem poised to accelerate the blue-state movement toward a lower-carbon economy. With the last two coal power plants in Oregon and Washington state scheduled to close after 2020, for instance, Pacific Northwest environmentalists already are pushing to ban electricity imports generated from coal, and to block coal exports to Asia. Their goal: Create the nation's first "coal-free zone," says Cesia Kearns, a Sierra Club representative.