Donald Trump has promised to repeal two federal regulations for every new one issued. An obscure law can help him do it. And it comes courtesy of former House speaker Newt Gingrich.

In 1996, Gingrich, as part of his Contract with America, shepherded the Congressional Review Act into law, giving Congress the power to kill unwelcome federal regulations. It’s seldom used, but it’s expected to be dusted off early next year and used by the incoming US president to destroy rules issued by the Obama administration.

The act allows senators and representatives who disapprove of a regulation to enter a resolution eliminating it. Resolutions are treated differently from bills; they can’t be filibustered and need only a simple majority to pass. They still require the signature of the president, though, so the law is almost never invoked because the right political circumstances rarely occur: Generally speaking, one party needs to control both houses of Congress and the White House, with a president eager to undo the policies of his immediate predecessor.

“The Congressional Review Act is the Halley’s comet of the regulatory policy world,” says James Goodwin, a senior policy analyst at the Center for Progressive Reform, a left-leaning think tank. “It has fleeting significance every four to eight years.”

In fact, it’s only been successfully employed one time, to kill an Occupational Health And Safety Administration rule on ergonomics. That occurred in 2001, after George W. Bush took control of the White House from Bill Clinton. Republicans have sought to use the review act since then, but Obama has vetoed five resolutions attempting to remove rules issued by environmental, labor, and financial agencies.

With Republicans controlling Congress and Trump in the White House, lobbyists and analysts on both sides of the aisle expect it to be different this time. “I would say it’s almost a certainty that they use it,” says Steve Keen, a lobbyist with the National Federation of Independent Business.

The Obama rules at risk in a Trump administration

The Congressional Research Service has identified at least 50 major rules that might be vulnerable in the next Congressional session. Among the possible victims are regulations requiring the safe storage of natural gas and methane, a rule that federal contractors must provide sick leave to employees, and one that bars nursing homes receiving federal funds from forcing residents into binding arbitration to settle disputes.

It’s unlikely all, or even most, of the 50 identified rules will be targeted. Much of what happens will come down to timing.

Congress has 60 days to submit a disapproval resolution after a rule is issued by the administration. But if Congress adjourns within 60 legislative days—days Congress is in session—of the rule being published, the calendar resets in the next Congress, to prevent the White House from having an advantage in sneaking in midnight rules in the last days of its administration.

As a result, members of both parties are paying close attention to when rules are issued, and counting backward from when Congress is expected to gavel out. The earlier Congress finishes, the more rules could be subject to the Congressional Review Act.

“Until the House and Senate adjourn, and the parliamentarian makes a ruling on what’s the farthest back date, we won’t know what’s in the bucket,” Keen says.

Based on the Senate’s targeted last day of Dec. 16, only rules issued after May 30 could be removed by resolution. If Congress were to gavel out a week earlier, they window could extend back to May 23, which is when the Obama administration issued its final rules extending overtime to as many 4.2 million workers. That rule, scheduled to take effect Dec. 1, was blocked by a judge who issued an injunction last week; the review act could eliminate the need for further litigation.

The real-world impact of rule rollbacks

Repeals can have real consequences for real people. Part of the Congressional Review Act says that after a rule is removed, similar ones can’t be issued to replace it. Federal agencies that over-interpret the law might avoid making any further rules on the topic, says Goodwin. He points to Clinton’s ergonomics rule overturned by the CRA. Thousands of poultry workers suffer from serious carpal tunnel injuries that could be prevented by workplace ergonomics rules, but OSHA has effectively abandoned the cause since the 2001 rule rollback, Goodwin says.

Regulations are often the result of years of diligent research. In that sense, the CRA “replaces what’s best about policy making with what’s worse, these kind of shoot-from-the-hip political statements from Congress,” Goodwin argues. ”Anyone who cares about good government should find that objectionable.”