Kevin J. McIntyre, right, in May 2018. (Kristoffer Tripplaar/For The Washington Post)

Kevin J. McIntyre, who chaired the Federal Energy Regulatory Commission and last year rejected a Trump administration proposal to aid coal and nuclear plants, died Jan. 2 at his home in Arlington, Va. He was 58.

The cause was brain cancer, his family said.

Mr. McIntyre, a Republican lawyer, was nominated by President Trump in August 2017 to chair the FERC, which regulates portions of the electricity grid, interstate oil transportation and natural gas pipelines, and electric transmission lines.

But last January, Mr. McIntyre led the independent five-member commission in unanimously rejecting a proposal by Energy Secretary Rick Perry that would have propped up nuclear and coal plants struggling to stay afloat in competitive electricity markets.

Mr. McIntyre’s death opens up the possibility that Trump could seek to alter the balance of the commission on competitive markets. The commission, which has two Democrats and two remaining Republicans, is also charged with approving utility mergers and the construction of liquefied natural gas facilities.

The Trump plan that the FERC rejected was widely seen as an effort to skew competitive electricity markets that federal regulators have been cultivating since the late 1980s. Critics said it would have largely helped a handful of coal and nuclear companies, including the utility FirstEnergy and coal-mining firm Murray Energy, while raising rates for consumers. Murray’s chief executive, Robert Murray, has been a major backer of Trump.

Kevin J. McIntyre. (Kristoffer Tripplaar/For The Washington Post)

Since then, the administration has been considering another proposal that would urge the FERC to invoke its rarely used emergency powers under the Federal Power Act to prop up the same coal and nuclear plants. Mr. McIntyre seemed unconvinced.

“I don’t think we have an emergency on our hands right now in the sense of the ability of our grid to perform today and in coming weeks and months,” McIntyre said at a Washington Post Live event in May. “It’s perhaps not the most obvious fit.”

He said the Federal Power Act “tees off of the concept of continuance of a war in which the United States is involved as being kind of the baseline circumstance that would justify a DOE order to certain types of facilities to either begin operating or continue operation.”

Mr. McIntyre’s stance at the FERC was something of a surprise to many. A graduate of San Diego State University and Georgetown University Law School, he spent most of his 30-year legal career at the Washington office of Jones Day, where he became co-leader of the firm’s energy practice.

A variety of environmental groups initially opposed Mr. McIntyre’s nomination, pointing to his work for a dozen or so energy companies. They included a supplier of coal to the nation’s largest coal-fired power plant; utilities in Ohio, Missouri, Florida and South Carolina; oil and gas exploration firms; and pipeline companies.

But on Thursday, John Moore, a senior attorney and FERC expert at the Natural Resources Defense Council, said in a statement that Mr. McIntyre “led FERC with a steady hand and with an emphasis on preserving open electricity markets and maintaining the independence of the commission. We especially salute his high civic calling.” Moore hailed Mr. McIntyre for “fairness and evenhandedness.”

Sen. Lisa Murkowski (R-Alaska), chairman of the Senate Committee on Energy and Natural Resources, said in a statement that “as a lawyer, a commissioner, and as FERC’s chairman, he always had the utmost respect for the agency and its mission.”

Mr. McIntyre did, however, lead 3-to-2 decisions along party lines in favor of natural gas pipelines. He also launched a close look at the “resilience” of the electricity grid.

The remaining members of the commission are Democrats Cheryl A. LaFleur and Richard Glick and Republicans Neil Chatterjee, a longtime staffer for Senate Majority Leader Mitch McConnell (R-Ky.), and Bernard L. McNamee, who was narrowly confirmed despite controversy over comments he made criticizing renewable energy. McNamee also said that the energy industry’s legal fights with environmental groups were a “constant battle between liberty and tyranny.”

Chatterjee, who became the FERC chairman in October, has been widely seen as a reliable supporter of Trump policy.

But in a briefing for journalists on Oct. 31, Chatterjee said that his view of FERC’s role was changing thanks to Mr. McIntyre, who he said “so emphasized the importance of the rule of law.”

Chatterjee added that Mr. McIntyre “could not be more strenuous in saying that politics should not be allowed to interfere with the work of the commission and that has really helped me grow in my role as I’ve made the transition from formerly partisan legislative aide to independent regulator. I credit one individual for enabling that growth and that’s Kevin McIntyre.”

Kevin Joseph McIntyre was born Dec. 27, 1960, at Dover Air Force Base in Delaware and moved frequently along with his father’s assignments in the military.

Survivors include his wife, Jennifer Brosnahan McIntyre, and their three young children, Elizabeth, Thomas and Anna, all of Arlington; his parents, Jack and Alice McIntyre of McLean, Va.; and four siblings.

Mr. McIntyre’s family said he was passionate about music and theater. He sang in productions of the Georgetown Gilbert & Sullivan Society, including the starring role as the Pirate King in “The Pirates of Penzance.”

He was “wholly invested in all things Celtic,” his siblings said in a note, and he was active in the Cathedral of St. Thomas More in Arlington. An avid runner, he completed five marathons.