The Environmental Protection Agency just imposed what may become the first federal mandate to cut carbon dioxide from power plants — if it survives legal and political hurdles that has tripped up past administrations.

Why it matters: Electricity is among the largest sources of U.S. carbon emissions. The country's power mix has trended away from coal for years as gas and renewables gained market share.

• Emissions are already falling, so the new rules are aimed at accelerating the trend.

Catch up quick: An Obama-era power plant CO2 rule — one very different from Biden's current plan — and a more modest Trump-era plan were both halted in court before they were able to take effect.

The big picture: The intricate matrix of CO2 standards covers today's fleet of coal-fired power plants, and natural gas-fired plants built in the future.

• EPA estimates the rule, alongside the 2022 climate law, will cut power plant emissions by 75% relative to 2005 levels by 2035, and keep going.
• It projects $370 billion in cumulative net climate and public health benefits through 2047.

In a burst of electricity policymaking, the EPA also unveiled rules covering air toxic emissions, wastewater from coal plants, and management of waste "coal ash."

How it works: Under the CO2 rule, some plants must capture carbon emissions to comply — using technology that has made little progress in the power sector to date.

• For instance, coal plants slated to keep running beyond 2039 have CO2 capture requirements that begin in 2032.
• Meanwhile, rules for emissions cuts from new gas plants vary based on how heavily they're operated.
• "Baseload" gas plants — those reaching at least 40% of their annual capacity — require carbon capture starting in 2032.

Flashback: The biggest change from last year's proposal is that it no longer covers existing gas-fired plants — the biggest U.S. power source.

• They're slated to be addressed in a subsequent rule, the EPA said weeks ago. But the long process effectively requires President Biden to win reelection.

The intrigue: As they impose new mandates, Biden officials are emphasizing financial carrots (like tax credits in the 2022 climate law) and other complementary policies to ease the deployment of climate-friendly power.

• The Energy Department is also completing rules to boost interagency coordination on transmission permitting and paring back reviews needed for projects on existing rights of way.

What they're saying: Activists generally cheered the new climate and environmental rules. However, views differ on the decision to defer CO2 emissions from current gas-fired power plants to a subsequent plan.

• The World Wildlife Fund called this "deeply concerning" and said it leaves "significant uncertainty."
• Meanwhile, the U.S. Chamber of Commerce said the CO2 rule will "significantly restrict electricity supply" even as more power is needed to meet rising demand from data centers, manufacturing and more.

What's next: Even if Biden is reelected, the power plant CO2 rules could face court challenges.

• The current Supreme Court looks askance at big executive decisions that Congress has not explicitly authorized.