The American Petroleum Institute, Washington’s biggest lobby for the oil-and-gas industry, spent decades leveraging its financial muscle to fight almost every green initiative in its path.

Then in March, the group signaled an about-face. It released its “Climate Action Framework,” a set of new policy prescriptions to lower emissions and support cleaner fuels.

The core of the plan called for two policies API had opposed for years: more regulation on methane, a potent greenhouse gas that leaks from oil-and-gas operations, and a price on carbon, a financial penalty levied on all carbon-dioxide emissions.