A heated New Hampshire primary in the state’s First Congressional District is going out with a literal bang.

Karoline Leavitt, 25, who served an assistant in President Donald J. Trump’s press office and turned the primary into a bitter battle over which candidate carried the mantle of Trumpism, is closing her campaign Monday night with a gun shoot at the Londonderry Fish & Game Club in Litchfield, N.H.

Special guests include two Republican members of Congress, Lauren Boebert of Colorado and Byron Donalds of Florida, Freedom Caucus members who represent the hard right wing of the party that Ms. Leavitt has aligned herself with. Ms. Boebert is pairing the campaign event with a signing of her memoir, “My American Life.”

The event was set to be a splashy coda for a candidate who gained traction with conservative voters by reaching for the most extreme and provocative statements and molding herself in the image of Mr. Trump.

The candidate who appears to be ahead, Matt Mowers, 33, is also a former Trump administration official billing himself as an “America First” conservative. He was set to spend the final night of his campaign visiting bars and restaurants in Manchester, a campaign aide said. The two former Trump aides are vying for the chance to run against the incumbent, Representative Chris Pappas, a Democrat.

The tight race between Ms. Leavitt and Mr. Mowers divided MAGA Republicans and the party’s leaders in the House. The minority leader, Representative Kevin McCarthy of California, and former Trump campaign aides like Corey Lewandowski and David Bossie endorsed Mr. Mowers. Representative Elise Stefanik of New York, who is the No. 3 House Republican, and Senator Ted Cruz of Texas were among those who endorsed Ms. Leavitt.

Mr. Mowers, who won the Republican nomination for the same House seat in 2020 but lost to Mr. Pappas, entered the race a year ago as the presumed front-runner. But Ms. Leavitt mounted a strident and surprisingly fierce challenge by billing herself as the anti-establishment candidate and savaging her opponent as a creature of the political “swamp.”

A recent poll by the University of New Hampshire Survey Center showed Mr. Mowers leading Ms. Leavitt by a thin margin: 26 percent to 24 percent, barely more than the 2.2 point margin of error, though 26 percent of likely voters said they remained undecided.

“Karoline Leavitt has been the straw that stirred the drink,” said Dante J. Scala, a political science professor at the University of New Hampshire.

In campaign mailers, Mr. Mowers touted his 2020 endorsement from the former president, who did not endorse either of his former aides in this year’s race.

The Congressional Leadership Fund, a super PAC aligned with Mr. McCarthy, has spent more than $1.3 million supporting Mr. Mowers. Another super PAC that supports moderate Republicans, Defending Main Street, has spent over $1.2 million and is running an ad that describes Ms. Leavitt as a “woke Gen-Z’er” and plays a Snapchat video she once posted where she used crude language to refer to her viewers.

The race will be decided Tuesday night.