THE GREEN DEAL IS OLD:

In Maine, the new governor, Janet Mills, a Democrat, has vowed to restore incentives for rooftop solar and to boost wind power locally -- moves that had been stymied by her Republican predecessor.

In New Mexico, another Democrat, Gov. Michelle Lujan Grisham, is backing a bill requiring electric utilities to get 50 percent of their power from renewable sources by 2030, keeping pace with neighbors like Colorado and Nevada. (Nevada voters in November approved their own requirement for 50 percent renewables by 2030.)

The most striking development, though, has been the array of governors who are now floating plans for their states to get 100 percent of their electricity from zero-carbon sources. Legislators in California and Hawaii have already set deadlines for utilities to meet this target by 2045. In recent months, the governors of Colorado, Illinois, New Jersey and New York have pledged to pursue similar goals.

These states are all venturing into uncharted territory, and there's no guarantee they will succeed. As states rely on ever-larger amounts of wind and solar power, it becomes more challenging to juggle these intermittent sources. Getting all the way to 100 percent zero-carbon electricity, experts say, could require extensive new nationwide transmission lines, novel energy storage techniques or help from untested technologies like advanced nuclear power.

For now, states are experimenting with varied approaches. Hawaii, for example, wants to meet its goal entirely through renewable energy. In New Jersey, by contrast, Gov. Philip D. Murphy signed legislation to keep his state's nuclear plants open as part of a broader low-carbon portfolio. And New York is soliciting bids for large new offshore wind farms.

Electricity is responsible for about one-third of America's carbon dioxide emissions. To go further, states will also have to clean up the cars and trucks on their roads, which account for another third.

In December, nine Eastern states and the District of Columbia announced they would work together to put a price on emissions from transportation fuels and invest the revenue in lower-carbon solutions, potentially including mass transit, electric buses or new charging stations to make it easier for people to own plug-in vehicles.

Some of the states involved, like Pennsylvania and Maryland, are in danger of missing their self-imposed climate goals unless they can halt the stubborn rise in driving emissions.

While the finer details of the policy will be hashed out this year, the states are modeling their efforts after the Regional Greenhouse Gas Initiative, a cap-and-trade system in the Northeast that auctions a steadily dwindling supply of carbon pollution permits to power plants and uses the revenue to invest in efficiency and clean energy programs.

"Transportation is going to be even more complex than electricity -- there are so many moving parts," said Vicki Arroyo, the executive director of the Georgetown Climate Center, which has been working closely with the states on the initiative. But, she said, referring to Maryland, Massachusetts and Vermont, "It's notable that we have three Republican governors here who are committed to stepping up on this."

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