As President Trump moves unilaterally to slash the federal bureaucracy and upend longstanding policies, Republicans in Congress have embarked on a spree of deregulation, using an obscure law to quietly but steadily chip away at Biden-era rules they say are hurting businesses and consumers.

In recent weeks, the G.O.P. has pushed through a flurry of legislation to cancel regulations on matters large and small, from oversight of firms that emit toxic pollutants to energy efficiency requirements for walk-in freezers and water heaters.

To do so, they are employing a little-known 1996 law, the Congressional Review Act, that allows lawmakers to reverse recently adopted federal regulations with a simple majority vote in both chambers. It is a strategy they used in 2017 during Mr. Trump’s first term and are leaning on again as they work to find ways to steer around Democratic opposition and make the most of their governing trifecta of the House, the Senate and the White House.

But this time, Republicans are testing the limits of the law in a way that could vastly expand its use and undermine the filibuster, the Senate rule that effectively requires 60 votes to move forward with any major legislation.

Because resolutions of disapproval under the Congressional Review Act need only a majority vote, they are some of the only legislation that can avoid a filibuster in the Senate. This allows them to circumvent the partisan gridlock that stands in the way of most significant bills.

Senator John Thune of South Dakota, right, the majority leader, with Speaker Mike Johnson. Mr. Thune must decide whether to go forward with a move that would undermine the filibuster.Credit…Haiyun Jiang for The New York Times