Leavitt Upsets Mowers, Winning New Hampshire House G.O.P. Primary

Karoline Leavitt, a 25-year-old hard-right Republican who served as an assistant in President Donald J. Trump’s White House press office, won her party’s nomination on Tuesday for New Hampshire’s First Congressional District, according to The Associated Press.

The race had devolved into a nasty battle with Matt Mowers, a former Trump administration colleague, over who carried the mantle of Trumpism.

“Unfortunately, tonight’s results did not go our way,” Mr. Mowers wrote in a concession statement on Twitter at 11:25 p.m.

Ms. Leavitt’s upset victory means she will face off in November against Representative Chris Pappas, a two-term Democratic congressman representing the highly competitive district in the eastern and southern parts of the state. Mr. Pappas is one of the most vulnerable Democrats this cycle, with his re-election race considered a tossup.

If she wins, Ms. Leavitt would be among the youngest people ever elected to Congress. The Constitution requires House members to be at least 25 years old to serve. Ms. Leavitt turned 25 last month.

Ms. Leavitt defeated Mr. Mowers, 33, a veteran of Mr. Trump’s State Department and of his 2016 campaign, who entered the race a year ago as the presumed Republican front-runner and benefited from an infusion of cash from an outside PAC aligned with Representative Kevin McCarthy, the Republican from California and minority leader, who is campaigning to become speaker.

The candidates have few discernible differences on policy, and the race ultimately turned less on any ideological divide than on style and tone. It divided the House Republican leadership, exposing lingering rifts inside the party over Mr. Trump’s influence.

Ms. Leavitt, who adopted Mr. Trump’s brash style and taste for inflammatory statements, was backed by a host of hard-right Republicans in Congress, most notably Representative Elise Stefanik of New York, the No. 3 Republican, who has also styled herself in the former president’s image. In her campaign, Ms. Leavitt unequivocally repeated Mr. Trump’s lie that the 2020 presidential election was stolen.

Ms. Leavitt leaned into the attacks and the money pouring into the district to defeat her, positioning herself as the America First candidate fighting “the swamp” and claiming that if “establishment Republicans” were spending so much to defeat her then “I must be doing something right.” Tucker Carlson, the Fox News host, also elevated her as the anti-establishment candidate in the race.

The Congressional Leadership Fund, a super PAC aligned with Mr. McCarthy, spent more than $1.3 million supporting Mr. Mowers. Another super PAC that supports moderate Republicans, Defending Main Street, spent over $1.2 million to attack Ms. Leavitt.

Ms. Leavitt’s come-from-behind victory was also a win for Ms. Stefanik, who harbors ambitions to rise in the party. Her outside group E-Pac, which supports conservative female candidates, spent the legal maximum, $10,000, supporting Ms. Leavitt’s campaign. Ms. Stefanik also served as an informal adviser to Ms. Leavitt, who previously worked as her communications director.

If Republicans win back control of the House of Representatives and Ms. Leavitt wins her seat in November, she could be a wild card for Mr. McCarthy in the mold of Representative Marjorie Taylor Greene of Georgia and other hard-right lawmakers who have sometimes proved a thorn in the minority leader’s side.

But Ms. Leavitt enters the general election bruised by the bitter primary. Mr. Mowers’s campaign operated a website branding her “fake MAGA Karoline.” It accused her of having “never held a real job outside the swamp,” attending private school in Massachusetts and being registered to vote from the “penthouse” apartment where she lived in Washington before moving back to New Hampshire to run for office.

And Ms. Leavitt’s brash style may make for an easier target for Democrats in a general election.

“I think she is more beatable because Democrats can portray her as an inexperienced ideologue,” said David Wasserman, an election expert with the Cook Political Report, a nonpartisan newsletter that analyzes elections.