WASHINGTON — Just days after the November election, top aides to Donald J. Trump huddled with congressional staff members in Speaker Paul D. Ryan’s suite of offices at the Capitol. The objective: not to get things done, but to undo them — quickly.
For about three months after Inauguration Day, Mr. Trump would have the power to wipe away some of his predecessor’s most significant regulations with simple-majority votes from his allies in Congress.
But the clock was ticking.
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An obscure law known as the Congressional Review Act gives lawmakers 60 legislative days to overturn major new regulations issued by federal agencies. After that window closes, sometime in early May, the process gets much more difficult: Executive orders by the president can take years to unwind regulations — well beyond the important 100-day yardstick for new administrations.
So in weekly meetings leading up to Jan. 20, the Trump aides and lawmakers worked from a shared Excel spreadsheet to develop a list of possible targets: rules enacted late in Barack Obama’s presidency that they viewed as a vast regulatory overreach that was stifling economic growth.
The result was a historic reversal of government rules in record time. Mr. Trump has used the review act as a regulatory wrecking ball, signing 13 bills that erased rules on the environment, labor, financial protections, internet privacy, abortion, education and gun rights. In the law’s 21-year history, it had been used successfully only once before, when President George W. Bush reversed a Clinton-era ergonomics rule.
The effort has surpassed its architects’ most ambitious hopes. Andrew Bremberg, the president’s domestic policy chief, said he had thought Congress might be able to use the act to pass five or six bills reversing Mr. Obama’s regulations. During the transition effort, no one contemplated more than a dozen, Mr. Bremberg said.
“It is a strong and very potent and powerful tool,” he said.
But critics say Mr. Trump’s aggressive use of the Congressional Review Act amounts to a blunt and thoughtless assault on rules that would have increased people’s safety, secured their personal information, protected federal lands and improved education.
The first Obama rule that Mr. Trump and Congress reversed would have required coal companies to make sure that waste from mountaintop mining was not polluting local waterways. Now, steps to prevent illness from contaminated drinking water will not be taken.
Another rule would have required the Social Security Administration to provide information about severely mentally incapacitated people to law enforcement agencies that conduct background checks for gun purchases. Now, these individuals — an estimated 75,000 a year — will not need Justice Department waivers to buy guns.
One eliminated regulation would have prohibited internet providers from collecting, sharing or selling consumers’ information without their permission. One would have required some businesses to keep records of work-related injuries and illnesses for five years instead of six months. And another would have prevented states from denying funding for women’s health services to facilities that also provided abortions.
Republicans viewed those rules and the other eliminated regulations as unnecessary burdens enacted by a president who had resorted to executive action because he could not get his agenda through Congress. While initially skeptical of using administrative power to govern, Mr. Obama increasingly embraced the use of regulation, reshaping government more by writing new rules than by passing new legislation.
“The biggest frustration in the last eight years was not knowing where the next regulation was coming from, the next rule, and that uncertainty stifled investment,” said Marc Short, Mr. Trump’s legislative affairs director, who participated in planning for the regulatory assault.
Mr. Trump’s efforts to unwind Mr. Obama’s regulations go beyond the use of the Congressional Review Act. He has issued executive orders, including one instructing the Environmental Protection Agency to begin the process of rolling back far-reaching rules that would shut down many of the country’s coal-fired power plants.
But reversing regulation through executive authority requires long periods of study, notice to the public, and hearings. The final outcome is often challenged in court, adding to the delay.
Under the Congressional Review Act, the process is cleaner and simpler. It requires only an up-or-down vote, and the outcome cannot be challenged legally.
The use of that tool to attack Obama-era regulations was coordinated by a small group, including Mr. Short; Mr. Bremberg; Eric Ueland, a veteran Republican who works for the Senate Budget Committee; Rick Dearborn, the director of the transition effort and now a deputy chief of staff at the White House; and House and Senate aides. The group’s members settled on a list of rules they thought could be eradicated.
“We knew we had a short window of time in order to do them,” said Don Stewart, a spokesman for Senator Mitch McConnell of Kentucky, the majority leader. “That was an important part of the coordination effort.”
Many Obama-era rules may survive Mr. Trump’s efforts to unwind them. Republicans have yet to repeal the Affordable Care Act, for which many of the most significant rules were written. Still, Mr. Trump’s critics say he has set a dangerous precedent with what they call his indiscriminate use of the Congressional Review Act.
The critics are especially concerned about a key provision in the act that seeks to prevent all future presidents from replacing the eliminated regulations with anything similar. That part of the act has never been tested in court, but experts said it would chill efforts to draft new regulations even after Mr. Trump leaves office.
“The Congressional Review Act used in this way is kind of like a nuke,” said Robert Hahn, a professor of economics at Oxford and an expert on American regulations. “We had a Democratic president who was reflecting his policy preferences toward regulation. Trump has a tool now to undo those political preferences, and he’s using it.”
Public Citizen, a liberal watchdog group, said in a statement that Mr. Trump and congressional Republicans “have been using the C.R.A. to reward the corporate and ideological special interests that funded their campaigns. It’s an escalation of the corrupt insider-dealing that Trump campaigned against.”
But what Democrats viewed as important new protections, Republicans saw as unneeded encumbrances on insurance companies, banks and other businesses.
“It’s not as if there aren’t an enormous number of regulations still on the books,” Mr. Short, the president’s legislative affairs director, said. “I don’t think that we feel like there is some sort of threat by passing this legislation.”
He added, “I think it would be unfair to paint it as if you are moving into an anarchical society.”