May 2006. Mr. Romney, the Massachusetts governor, was chatting up his Montana counterpart about his vision for an energy-efficient car of the future — lightweight and narrow, with tandem-style seating, so that two vehicles could drive side by side in one highway lane.

“He was getting animated about all these little cars that would be driving around in these lanes,” Brian Schweitzer, the Montana governor, recalled in an interview. Mr. Schweitzer, a Democrat, laughed, thinking about how Republicans might react. “I said, ‘Mitt, if you ever run for president, don’t ever talk about that again.’ ”

Today, as the Republican nominee for president, Mr. Romney is far more apt to talk about oil drilling than energy-efficient cars. He has presented a plan to open up more land and coastline to oil and gas drilling, grant speedy approval to the Keystone pipeline to transport crude oil from Canada to the United States, end wind and solar power subsidies and curb regulations that discourage burning coal for electricity. It is an agenda far different from the one he outlined in his early days as governor.

He populated his Massachusetts administration with environmentalists, including one, Gina McCarthy, who now runs the clean air division of the Environmental Protection Agency under President Obama. He railed against the “Filthy Five,” high-polluting power plants in the state. He issued a “climate protection plan” and lauded it as “among the strongest in our nation.” Under his direction, Massachusetts helped create a regional cap-and-trade program — anathema to most Republicans — intended to cut the greenhouse gas emissions that scientists believe cause global warming.

But at the last minute, Mr. Romney refused to sign the greenhouse gas pact that his aides had spent more than two years negotiating; industry opposed it, and some former Romney policy advisers say his political team feared that it would doom his chances for the presidency. A current campaign policy adviser, Oren Cass, said Mr. Romney simply evaluated the proposal and “concluded the costs imposed on the economy would be too high.”

Today in Massachusetts, environmentalists credit Mr. Romney with helping to promote smart growth and reducing air pollution by putting in place tough regulations curbing certain toxic emissions from power plants. They also praise him for signing into law a bill embracing oil spill prevention measures. But many feel betrayed by his surprise reversal on the climate change pact.

“He was ahead of his time and very progressive,” said Jack Clarke, who was appointed to an ocean management task force by Mr. Romney and now directs public policy at Mass Audubon, a conservation group. “But by the end of his administration, he seemed to have gotten Potomac fever. The conservation agenda he had didn’t seem to be a core conviction.”