The measure moves energy generated by burning trash into a top-tier category for renewable energy.
More than 35 local, state and national organizations sent a letter to O'Malley Tuesday pressing for a veto. Critics say the measure would degrade the standard designed for cleaner energy, such as wind and solar power.
O'Malley told reporters on the last day of the legislative session in April that he is in favor of "next-generation waste-to-energy."
He said his administration initially supported the bill, but he noted he had not seen the final version of it. A spokesman for the governor says that O'Malley is still considering the measure.
The state has a goal of generating 20 percent of its energy from "clean, renewable sources" by 2022. The Frederick News Post reports that this legislation could help the state reach its goal because it would broaden the definition of renewable energy.