WASHINGTON — Climate change was supposed to be one of Michael R. Bloomberg’s greatest strengths in the Democratic presidential primary.

Where other candidates have plans to reduce fossil fuel use, the billionaire philanthropist has results: millions of dollars spent to help close hundreds of coal-fired power plants and, in many cases, to secure a switch to natural gas, which produces about half the planet-warming greenhouse gases as coal.

But that transition, once considered a bragging right in the environmental world, has become a liability to his campaign at a moment when the only acceptable fossil fuel is no fossil fuel.

As he seeks to win over climate-conscious Democrats, Mr. Bloomberg has found himself unable to shake the incrementalist label in a maximalist primary. During his debut debate in Nevada, Mr. Bloomberg allowed that an economy powered solely by wind, solar and other renewable energy sources was “still many years from now.” Hydraulic fracturing for natural gas, or fracking, will continue “for a while,” he said, as he promoted natural gas as a bridge between coal and carbon-free power.