Long before the League of Conservation Voters embraced him as the “greenest governor” in America, Jay Inslee of Washington had written a book about climate change that called for a clean-energy revolution that would be the equivalent of the Apollo space program in commitment and innovation.

But Mr. Inslee’s visions for advancing some of the world’s most ambitious climate policies repeatedly ran into the realities of politics during his 12 years in office in Olympia, the state capital. Even in a largely liberal state where residents cherished the environment, legislators were reluctant to get on board. In 2018, voters rejected a carbon tax that Mr. Inslee had championed.

It wasn’t until last year that Washington became the second state in the country to implement a landmark law capping emissions, requiring businesses to pay for the right to generate large quantities of carbon dioxide.

Now, as Mr. Inslee prepares to leave the governor’s mansion at the end of his third term, he is helping lead the fight against a ballot initiative, well-funded and fiercely fought, that would repeal Washington’s climate law.

“This is the single most important election, other than the White House, in the United States,” Mr. Inslee told supporters at a union hall this month in Everett, about 25 miles north of Seattle.

The vote on the measure, Initiative 2117, is being watched across the country and around the world as government leaders struggle with how to put in place plans with enough potency to drastically alter carbon emissions — the primary cause of global warming — while answering to voters concerned about the costs.

Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.