Representative Elise Stefanik of New York has been one of President-elect Donald J. Trump’s most ferocious allies for years, defending him during his two impeachments, amplifying his lies about the 2020 election and rallying behind his comeback campaign.

On Monday, her loyalty paid off: In one of the first major personnel announcements for his second term, Mr. Trump said he would nominate Ms. Stefanik as his ambassador to the United Nations.

Ms. Stefanik, 40, would bring relatively little diplomatic or foreign policy experience to the role, beyond having served as a member of House national security committees.

But in five terms representing the North Country of New York, she has made history several times over, beginning a decade ago, when she became the youngest woman ever elected to Congress at the time. Ms. Stefanik is currently the most senior woman in House Republican leadership, serving as chair of the House Republican Conference.

In recent months, her viral face-offs with Ivy League presidents over rising antisemitism on their campuses helped cause several to lose their jobs. A staunch supporter of Israel, she has also accused the U.N. of being plagued by “antisemitic rot.”

An establishment acolyte turned MAGA warrior

Few figures have better embodied the rapid evolution of the Republican Party under Mr. Trump than Ms. Stefanik.