President Trump on Wednesday threw the weight of the federal government behind vehicles that burn gasoline rather than electric cars, gutting one of the country’s most significant efforts to address climate change and thrusting the automobile industry into greater uncertainty.

Flanked by executives from major automakers in the Oval Office, Mr. Trump said the Transportation Department would significantly weaken fuel efficiency requirements for tens of millions of new cars and light trucks. The administration claimed the changes would save Americans $109 billion over five years and shave $1,000 off the average cost of a new car.

The Biden administration’s stricter efficiency standards were designed to get more Americans to go electric. But Mr. Trump said they “forced automakers to build cars using expensive technologies that drove up costs, drove up prices, and made the car much worse. This is a green new scam, and people were paying too much for a car that didn’t work as well.”

For the past half-century, the efficiency standards have compelled automakers to increase the distance their vehicles can travel on a gallon of gas, reducing fuel consumption and leading to innovations like electric and hybrid cars.

The announcement on Wednesday was the second part of a one-two punch against policies promoting electric cars, a central pillar of President Joseph R. Biden Jr.’s strategy for fighting climate change.

Transportation is the largest source of greenhouse gases in the United States, and Mr. Biden had adopted a carrot-and-stick approach to reducing these emissions. He offered tax credits to encourage motorists to buy electric cars while requiring that automakers meet stringent fuel efficiency standards to pressure them to sell more nonpolluting models.

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