Carbon prices are usually designed to raise the cost of fossil fuels in order to goad consumers into using them less. Under Initiative 1631, the price of gas would likely rise by 13 cents a gallon in 2020, and the price of home-heating oil would rise by 15 cents a gallon, according to the nonpartisan think tank Resources for the Future.

The initiative’s supporters say that—in a worst-case scenario—the policy will cost most Washingtonians about $10 a month. And they contend that companies will not succeed in transferring every cent of costs to consumers.

Carbon prices have been successfully implemented both in and outside of the United States. For almost a decade, California has charged polluters for the right to emit carbon pollution. But it uses a “cap and trade” scheme to set its carbon price, which means it uses a market to determine the price of emitting a ton of carbon dioxide. Under the law, the state’s annual carbon emissions have fallen 13 percent from their historical peak.

But carbon markets can fail, and Washington forgoes that approach. Initiative 1361 levies a single, economy-wide fee that will not gyrate with the swings of a market. No other state has imposed a carbon price in the same way. But elsewhere in the developed world, examples abound.

“Right next door to Washington, in British Columbia, there’s been a carbon tax for 10 years,” says Kristin Eberhard, a senior researcher at the Sightline Institute, an environmental think tank that focuses on the Pacific Northwest.

British Columbia’s tax is roughly twice as expensive as Washington’s proposed starting fee, and it rises faster than Initiative 1631 would increase. But the province’s carbon emissions have fallen by only 2 percent since 2007, when the fee was adopted—though this gap is mostly due to an enormous oil-and-gas exporting boom.

Canada on the whole will phase in a national carbon price of roughly $38 (in U.S. dollars) by 2022, though that plan could be canceled if Prime Minister Justin Trudeau is not reelected.

Roughly 20 percent of the world’s carbon emissions are covered by some kind of carbon-pricing scheme, according to the World Bank. But every single one of those policies was adopted or decreed by lawmakers. “This has never been done at the ballot,” Eberhard told me. “If this passes, it would be a huge win for climate action. It would show that people are actually willing to vote for a tax—that’s how much they care about climate change.”

It would also show that voters will do what state lawmakers—at least in the United States—cannot. Since Trump’s election, progressives have claimed that states and cities will take up the banner of climate action. But other than a series of vigorous lawsuits against Trump’s environmental rollbacks, blue-state Democrats have struggled to channel this resolve into new policy.