Democrats’ hopes of flipping Representative Elise Stefanik’s seat in New York were dealt a serious blow on Friday, when a Republican state assemblyman bowed to President Trump’s wishes and dropped his plans to run on the Conservative line against a Trump-endorsed candidate.

The assemblyman, Robert Smullen, said that he met with the president at the Oval Office, where Mr. Trump spoke of the importance of uniting Republicans and Conservatives before the midterm elections.

“I am deeply honored to have met with President Trump and grateful for our discussions,” he said.

The decision is a boon to Anthony Constantino, the chief executive of a novelty printing company called Sticker Mule, who won the Republican nomination decisively last month. Ms. Stefanik, who is stepping down at the end of the year, remained neutral in the primary, but she endorsed Mr. Constantino, a classmate from their private high school, shortly before the race was decided.

Mr. Smullen, a former marine and national security expert, had the support of local and state Republicans as well as the Conservative Party, whose ballot line would have allowed him to remain in the race.

But Mr. Constantino had the president’s support, winning Mr. Trump’s favor after he erected a massive “VOTE FOR TRUMP” sign atop his company headquarters. And he showered the president with extravagant tokens of his affection, including a life-size bronze statue of Mr. Trump and a self-produced rap album praising him.

Mr. Constantino highlighted his loyalty and his similarities to the president during his campaign. He also adopted the president’s practice of making crude personal attacks on social media, nicknaming his opponent “Slimeball Bob” and using A.I.-generated images to mock him. (Mr. Smullen appeared so personally repulsed by Mr. Constantino that he refused to shake his hand after a May debate).

Mr. Constantino adopted a conciliatory tone on Friday, thanking Mr. Smullen.

“It was a hard-fought campaign that sharpened my skills and taught me a lot,” he said in a statement. “I thank you for all your effort in this campaign and ongoing service to our nation.”

Mr. Smullen’s exit will make the race exceedingly difficult for the Democratic candidate, Blake Gendebien, a dairy farmer. His hopes for victory would have been buoyed had Mr. Smullen stayed in the race, potentially splitting voters on the right. The district is among the state’s most conservative, with 85,000 more Republicans registered than Democrats.

Since his primary victory, Mr. Constantino has earned the endorsements of House leadership, including Speaker Mike Johnson and the majority leader, Steve Scalise, He also has the support of roughly a dozen county chairs in his New York district who previously backed Mr. Smullen.

The Conservative Party could decide to replace Mr. Smullen on the ballot, but its chairman, Gerard Kassar, said that was unlikely. “The Conservative Party line will very likely remain blank,” he said on Friday.

Perhaps the most consequential late-arriving endorsement came from Ms. Stefanik, who remains very popular in the district.

“This was a hard fought election and the voters have spoken loudly and clearly,” Ms. Stefanik said.

Mr. Constantino will inherit Ms. Stefanik’s trusted right-hand man, Alex DeGrasse, a political operative who has worked closely with Ms. Stefanik and the National Republican Congressional Committee over the years.

His victory, as well as some of his campaign antics, has also earned some unwanted attention outside Republican circles.

Josh Johnson of “The Daily Show” did a segment on Mr. Constantino in which he mocked his anti-transgender and anti-solar-energy positions (“It’s like my uncle’s Facebook came to life”) as well as his musical ventures.

In January, Mr. Constantino released a 28-minute rap album, “Thank you President Trump.” It contains tracks titled “War,” “Family Man” and “Hole in One.” Joining Mr. Constantino on the album was Greatness, a Midwestern rapper who worked with Sticker Mule, Mr. Constantino’s company, to give away cash to strangers; and Roger Stone, the Trump associate and longtime Republican operative.

One track, simply titled “Trump,” is a list of countries that Mr. Constantino says “love Donald Trump.” The song is accompanied by an A.I.-created video that shows the president on a camel, skydiving, standing on the moon and moon-walking in New York.

In another music video Mr. Johnson played, Mr. Constantino lashes out at Mayor Zohran Mamdani of New York City, calling him a terrorist.

Mr. Constantino responded by falsely labeling Mr. Johnson of being nonbinary and noted that Mr. Mamdani’s birth country, Uganda, had some of the most violent anti-gay policies in the world. “I made the song to raise awareness so Zohran’s friends stop killing people like yourself. Despite being a dumb as rocks Communist, I still think you deserve the persuit of happiness,” he wrote on X.