For a President Romney to succeed in his pledges would require a significant revision to the Clean Air Act and other landmark environmental laws enacted over the past 40 years. It would also mean overcoming longstanding political opposition to oil drilling off the Florida coastline and in the Arctic National Wildlife Refuge. His administration would have to undergo a time-consuming process of withdrawing current and pending federal rules and resubmitting them through the complex regulatory system.

While Mr. Romney blames the Obama administration for the edifice of federal law and regulation that he argues is choking off economic recovery, many of these rules go back decades.

“It’s not just Obama he’s attacking, but past acts of Congress,” said Rena I. Steinzor, a specialist in administrative law at the University of Maryland and the president of the Center for Progressive Reform. “This does not all spring from the frenzied imagination of Obama’s E.P.A. It all comes down from statutes.”

She noted that the Clean Air Act was passed in 1970, and that while many conservatives and business lobbyists believe some of its provisions are costly and onerous, there is no groundswell to repeal it. Mr. Romney advocates changing one of its key provisions, requiring that human health standards be set without regard to cost, a shift supported by many House Republicans. But such a change would probably fail in the Senate if Democrats retained their majority there, and it would certainly be challenged in court.

The sharpest contrasts between a second Obama term and a first Romney term might come in the way air pollution and fossil fuels are treated. Mr. Obama postponed a decision in 2011 on a stricter new standard for smog-causing ozone pollution that would have thrown hundreds of cities out of compliance with clean air rules, requiring costly new plans for limiting pollution from transportation and industry. But the president said he intended to allow the rule to go forward in 2013 or 2014. Mr. Romney has said that the rule is too expensive and that he will not pursue it.