WASHINGTON -- Mitt Romney vowed in a campaign appearance earlier this year to "take a weed whacker" to the thicket of federal regulations adopted by the Obama administration and promised to impose a rigid freeze and cost cap on all new government rules.
He has pledged to reverse a half-dozen major Environmental Protection Agency pollution and public health rules, to swiftly approve the Keystone XL pipeline from Canada, to rewrite the ambitious new vehicle fuel efficiency standards and to open untouched coastal and wilderness areas to oil and gas exploration. Mr. Romney envisions a nation in which coal-burning power plants are given new life, oil derricks sprout on public lands and waters, industry is given a greater say in the writing and enforcement of environmental rules and the Code of Federal Regulations shrinks rather than grows.
But as President Obama learned in his first years in office as he tried to undo some of his predecessor's industry-friendly energy and environmental policies, such promises are easier to make than keep. They require the rewriting of decades-old laws, the assent of a balky Congress, favorable rulings from courts and a bureaucracy that works smoothly and swiftly to advance a president's goals. That is a steep hill in today's Washington.
Cutting the regulatory state down to size has been a mainstay of Republican campaign oratory for years, and the basis of an avalanche of bills passed over the past 18 months by the conservative House Republican majority, all of which have died in the Democratic-led Senate.
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.
The Obama administration has also signaled that it intends to regulate the disposal of coal ash from power plants and to continue efforts to limit mountaintop-removal coal mining, a particularly destructive form of extraction. Mr. Romney says such potential rules will hurt coal-producing regions and has promised to encourage coal production.
On hydraulic fracturing for oil and gas, which has led to a boom in domestic production, Mr. Romney has complained that federal rules are too strict and permitting is too slow. He wants to turn all permitting on public lands over to the states, an approach Mr. Obama has so far rejected because it would impose different standards across the country.
Mr. Romney has also said that he will reverse the Obama administration's proposed regulations limiting greenhouse gas emissions from power plants that contribute to climate change. In his acceptance speech at the Republican convention, he described such rules as an "assault on coal and gas and oil" that is driving up energy costs.
The Obama administration's efforts to address climate change arise from a finding that carbon dioxide and other heat-trapping gases endanger human health and the environment, based on the landmark 2007 Supreme Court decision Massachusetts v. E.P.A., brought by a dozen states, including Massachusetts when Mr. Romney was governor.
A federal appeals court panel recently upheld the rules adopted by the E.P.A. as a result of that ruling, saying that the agency's interpretation of the law and the Supreme Court decision were "unambiguously correct."
In responses to a recent questionnaire, Mr. Romney allowed that the world was getting warmer, that human activity contributes to the warming and that some action may be required to deal with it. But he opposes significant steps to combat climate change. He also has said he would revisit a recently announced fuel economy standard for cars and light trucks that would double fuel efficiency to an average of 54.5 miles per gallon for the 2025 model year while markedly reducing greenhouse gas emissions. The new target was the result of years of negotiation among automakers, the Department of Transportation, the E.P.A. and regulators in a number of states, led by California.
Mr. Romney characterized the new mileage goals as "extreme," saying they would limit vehicle choices for consumers and drive up the cost of new cars. Automakers, however, are not eager to renegotiate the standards and are already adopting designs and technology to meet them. The administration says that consumers will save as much as $8,000 in fuel costs over the life of the new vehicles, more than covering the added cost of the more efficient technology.
As part of his critique of the Obama administration's approach to regulation, Mr. Romney has asserted that the government has added tens of thousands of workers to write, review and enforce new federal rules.
In an address at the University of Chicago in March, Mr. Romney said that Americans' freedom is threatened by unelected, unaccountable regulators who he said were "always on the prowl."
"And under President Obama," he said, "they're multiplying like the proverbial rabbits. I mean, the number of federal employees has grown by 140,000 people under this president. And those regulators, they do a lot of damage."
Mr. Romney implied that all those new hires were regulatory jobs. In fact, almost all of the growth of the federal work force under the Obama administration has come in the Departments of Defense, Homeland Security and Veterans Affairs, according to PolitiFact, which based its analysis on data from the Bureau of Labor Statistics.
On one pending regulatory matter, the Keystone XL pipeline, Mr. Romney and Mr. Obama are likely to arrive at the same conclusion, although by different mechanisms and on different timetables. Mr. Romney has frequently said that on his first day in office he would approve the pipeline, a much-contested project to carry heavy oil from tar sands formations in Canada to refineries on the Gulf Coast.
Mr. Obama has approved a nearly 500-mile section of the pipeline from Oklahoma to the gulf and has signaled that he intends to green-light the rest of the project in 2013 after environmental and routing reviews are completed.
For Mr. Romney to make good on his pledge to approve the project on Jan. 20, 2013, he would need a determination from the State Department -- which would still be led by Secretary of State Hillary Rodham Clinton until noon on that day -- that the project was in the national interest. The department is not yet ready to issue that finding.
Correction: October 7, 2012, Sunday
This article has been revised to reflect the following correction: A picture caption with an earlier version of this article misidentified the building in the photo. It is the Treasury Department, not the White House.
This article originally appeared in The New York Times.