The Chamber of Commerce is seeking such changes. “We want to get this done before the election,” Mr. Johnson said. “The next White House may be less hospitable to our position.”

Image Craig L. Fuller, a lobbying executive, says businesses are adjusting their strategies. Credit... Susan Etheridge for The New York Times

Indeed, most of the Democratic candidates for president have offered proposals to expand the 1993 law, to provide paid leave and to cover millions of additional workers. Senator Christopher J. Dodd of Connecticut was a principal author of the law. Senator Hillary Rodham Clinton of New York says it has been “enormously successful.” And Senator Barack Obama of Illinois says that more generous family leave is an essential part of his plan to “reclaim the American dream.”

Susan E. Dudley, administrator of the White House Office of Information and Regulatory Affairs, said, “Research suggests that regulatory activity increases in the final year of an administration, regardless of party.”

Whoever becomes the next president, Democrat or Republican, will find that it is not so easy to make immediate and sweeping changes. The Supreme Court has held that a new president cannot arbitrarily revoke final regulations that already have the force of law. To undo such rules, a new administration must provide a compelling justification and go through a formal rule-making process, which can take months or years.

Within hours of taking office in 2001, Mr. Bush slammed the brakes on scores of regulations issued just before he took office, so his administration could review them. A study in the Wake Forest Law Review found that one-fifth of those “midnight regulations” were amended or repealed by the Bush administration, while four-fifths survived.

Some of the biggest battles now involve rules affecting the quality of air, water and soil.

The National Chicken Council and the U.S. Poultry and Egg Association have petitioned for an exemption from laws and rules that require them to report emissions of ammonia exceeding 100 pounds a day. They argue that “emissions from poultry houses pose little or no risk to public health” because the ammonia disperses quickly in the air.

Perdue Farms, one of the nation’s largest poultry producers, said that it was “essentially impossible to provide an accurate estimate of any ammonia releases,” and that a reporting requirement would place “an undue and useless burden” on farmers.