Gov. Gavin Newsom, a chief critic of the president and an opponent of oil exploration in the Pacific, called the proposal “dead on arrival.”

The Trump administration plans to allow new oil and gas drilling off the California coast for the first time in roughly four decades, according to three people briefed on the matter.

The move would set up a confrontation with Gov. Gavin Newsom, a Democrat who has fought offshore drilling and who has emerged as one of President Trump’s chief political antagonists. The governor is in Brazil for the United Nations climate summit, where he is drawing a contrast between himself and Mr. Trump, who denigrates efforts to fight global warming.

The Interior Department could announce the proposal as soon as this week, according to the three people, who spoke on the condition of anonymity because they were not authorized to comment publicly.

When asked about the proposal on Tuesday, Mr. Newsom rolled his eyes and said it would be “dead on arrival in California.” He said that the state would “absolutely” challenge the plan in court once it was finalized.

Speaking at a news conference in Brazil, he said it was “remarkable” that Mr. Trump did not call for drilling near Mar-a-Lago, the president’s Florida resort. “He didn’t promote it off the coast of Florida,” Mr. Newsom said. “That says everything about Donald Trump.”

There has been almost no fossil fuel development along the California coast since an enormous oil spill near Santa Barbara in 1969 that shocked the nation and helped to galvanize a national environmental movement. The spill sullied beaches in the popular coastal enclave, killed thousands of birds, damaged property and harmed the commercial fishing industry.

Since then, drilling has been prohibited in California state waters, which extend three miles from the shoreline. There has been limited drilling, and no new leasing, in federal waters off California since the mid-1980s.

The plan would also require new oil and gas leasing in the eastern Gulf of Mexico, which Mr. Trump is calling the Gulf of America. That could prompt pushback from Republicans from Florida, who have opposed new drilling there ever since the 2010 Deepwater Horizon rig disaster devastated the state’s tourism industry.

Elements of the proposal were reported earlier by The Washington Post and Politico. Representatives for the Interior Department and the White House did not immediately respond to requests for comment.