In one of his first cabinet-level personnel decisions, the president-elect has chosen the Republican member of Congress from New York to represent the United States at the United Nations.President-elect Donald J. Trump has offered Representative Elise Stefanik, Republican of New York, the role of U.N. ambassador in his upcoming administration.Ms. Stefanik, who represents an upstate New York district in the House and is a member of the Republican leadership in the chamber, has been a vocal supporter of Mr. Trump. His decision to name her to the post was reported earlier by CNN.Ms. Stefanik has accepted the offer, her office said.Ms. Stefanik, 40, emerged as a key ally to Mr. Trump during his first impeachment proceeding. She has been chair of the House Republican conference, but has minimal experience in foreign policy and national security. She has served on the House Armed Services Committee and the House Permanent Select Committee on Intelligence. She has been an outspoken supporter of Israel, and had a high-profile role in the congressional hearings that led to the resignations of several university presidents over their handling of campus unrest following the terror attack by Hamas on Israel and the subsequent war in Gaza.She also impressed Mr. Trump years ago with an outspoken defense of him during his first impeachment trial in the House. In a statement, Mr. Trump called her a “strong, tough and smart America First fighter.”House Republicans appear on track to win a narrow majority in the incoming Congress. Ms. Stefanik’s departure could make their margin even thinner until an election to replace her is held in what is considered a safe district for the party.It’s not yet clear whether Mr. Trump will be able to raid the House for his loyalists who serve there. Republicans are currently on track to keep their majority, but only by the similar razor-thin margin they have now, which has made it difficult to control the floor. Next year, they will be expected to produce major legislative results as a result of the party’s unified power in Washington.Ms. Stefanik, the first Trump ally from the House who has been announced as a cabinet pick, has long been positioning herself to rise in a Trump administration. But her situation may be particularly difficult. In New York State, Gov. Kathy Hochul, a Democrat, would most likely wait as long as possible to call a special election to fill her seat.Elon Musk, the billionaire and major Trump supporter, made clear on X that he had reservations about her appointment, based on the tight margin of control he is expecting in the House.“Elise is awesome, but it might be too dicey to lose her from the House, at least for now,” Mr. Musk wrote on the social media platform.Her selection comes after Mr. Trump last week named Susie Wiles, a longtime political operative who helped lead his campaign, as his White House chief of staff. On Sunday evening, Mr. Trump named Thomas D. Homan, an immigration hard-liner, to be his “border czar.”Annie Karni More