Voters in Washington state will be asked next month whether they want to adopt the nation’s first carbon tax – a powerful way to curb greenhouse-gas emissions. You’d think environmental groups would be doing everything they can to back that idea. You’d be wrong.

Initiative 732 will be on the ballot on Election Day. It calls for a $25-per-ton carbon tax, and it says how the proceeds should be spent: Trim the state’s sales tax, cut taxes on manufacturers and give tax rebates to low-income households.

Oddly enough, this plan has failed to impress some environmental campaigners. Washington Conservation Voters, an influential local organization, opposes it because none of the revenue will go toward funding clean energy. Climate Solutions, another local group, agrees. The Sierra Club won’t back the plan, saying its help for low-income households and minorities is insufficient.

Those groups haven’t put their own proposal on the ballot, so they’re saying it’s better to do nothing than vote for Initiative 732. This position is absurd. Curbing carbon emissions is, or ought to be, the primary goal, and the plan would do that. In addition, it’s an opportunity to prove the viability of the carbon-tax approach and set a valuable example for the rest of the country.

The part of the plan that seems to offend the campaigners – how the carbon-tax revenues ought to be used – is worth debating, but what matters most is to put a price on carbon, reflecting its true cost to the environment. Promising that the revenues from the carbon tax will be used to cut taxes and support low-income working families seems wise, in fact, because it lends the idea bipartisan appeal. But that isn’t the main thing. The main thing is that the measure shouldn’t fail merely because it doesn’t please every group on every point.

The latest polling shows 42 percent of respondents supporting the plan, with 37 percent against and 21 percent undecided. In other words, local groups could well decide the outcome, persuading undecided voters to lean one way or the other. It would be bad enough for the measure to be voted down because climate change wasn’t seen as an urgent problem. If it fails for lack of support from climate campaigners, that’s worse – and those groups should be ashamed.