If they succeed, from next year to 2030 they will have added a cumulative total of at least 620 million metric tons of carbon dioxide into the air, according to calculations by the Partnership for Public Integrity, an energy policy analysis group, based on a model used by the government’s Energy Information Administration to assess the impact of the Clean Power Plan.

That amounts to 48 million additional tons of carbon dioxide a year, on average, about three-quarters of what was produced by forest fires in the lower 48 states in 2013. It makes for a big hole in a plan that is supposed to cut annual emissions from the power sector by some 250 million tons between now and 2030.

The proposal not to count carbon from biomass is the work of Maine’s two senators — Susan Collins, a Republican, and Angus King, an independent — who introduced it into the Senate version of the energy bill passed earlier this year. It has broad support, and passed by a unanimous voice vote, according to those speaking for the chairman and ranking member of the Senate’s energy committee.

“The Senate has strongly supported the benefits that biomass utilization can have, including instances in which it is a carbon-neutral source of energy,” they wrote in a statement. “Conversations about the amendment are expected to continue among the bill conferees, the amendment sponsors and other congressional colleagues as the energy bill moves through the conference process.”

While it is hard to handicap its chances, the biomass proposal could become law within weeks. Members of Congress are now working to reconcile the Senate’s bill with the one that passed in the House, which does not include this provision. Even if it fails to make it through, similar language has been attached to appropriations bills for the Interior Department, passed by both chambers and now undergoing reconciliation.