The Republican architect of Project 2025 — the right-wing blueprint that Democrats made a rallying cry in the presidential election last year — is mounting a primary challenge to Senator Lindsey Graham of South Carolina, saying he isn’t sufficiently devoted to President Trump’s political movement.

As he begins his challenge, Paul Dans, who is not originally from South Carolina, starts out as a distinct underdog. Mr. Graham, who has the support of Mr. Trump, has won past primaries handily despite appearing vulnerable, and he is likely to have a significant financial edge.

But Mr. Dans plans to run highlighting the work of Project 2025, from which Mr. Trump distanced himself during his campaign before enacting significant portions of it into his government. And while Mr. Dans had nothing but praise for Mr. Trump, his candidacy comes at a moment when some of Mr. Trump’s hard-line allies have been critical of some decisions, including his authorization of military strikes in Iran and his handling of files in the Jeffrey Epstein investigation.

He intends to cast Mr. Graham as an old-guard Republican who is not aligned with Mr. Trump’s movement.

“He’s the very reason MAGA came into being,” Mr. Dans said of the president’s political movement. “He’s essentially anti-MAGA.” In an interview, he characterized Mr. Graham’s Senate record as producing “three endless wars and over $30 trillion in debt.” He added: “He has done so much damage to the movement.”

Mr. Trump in March announced his backing of Mr. Graham, with whom he has worked closely since his first term, declaring that he would not “let you down.” Chris LaCivita, who helped run Mr. Trump’s 2024 presidential campaign, is advising Mr. Graham’s campaign.

In a statement, Mr. LaCivita slammed Mr. Dans as having “parachuted himself into the state of South Carolina in direct opposition to President Trump’s longtime friend and ally in the Senate, Lindsey Graham. Like everything Paul Dans starts, this too will end prematurely.”

Mr. Dans is not the only Republican challenging Mr. Graham. The state’s lieutenant governor, André Bauer, announced his candidacy this month.

Mr. Dans would not say whether he expects major financial backing, and said he was focused on the grass roots. Yet Mr. Dans may draw unusual attention thanks to his association with Project 2025.

“The number of Democrat tears shed over Project 2025 would probably cure any drought in this country,” Mr. Dans said. “That said, they were right, they did see it as the existential event to the deep state” in the form of Mr. Trump’s aggressive approach to remaking government.

Mr. Dans, 56, grew up in the Baltimore suburbs and obtained two degrees at the Massachusetts Institute of Technology before studying law at the University of Virginia, where he was active in the conservative Federalist Society. He spent years in Manhattan, working at several corporate law firms, before starting his own firm.

“I’m an original Trump guy — I date back to the Trump Shuttle,” Mr. Dans said, saying he was an admirer of Mr. Trump from his early days as a rising businessman, which included operating an airline from 1989 to 1992.

After supporting Mr. Trump in his first presidential run, Mr. Dans hoped to land a job in the administration. That opportunity didn’t come until 2019, when he was hired as an adviser in the Housing and Urban Development department; six months later, he jumped to the Office of Personnel Management, the primary human resources agency of the federal government.

His signature achievement during that stint was helping enact Schedule F, a new job classification created in the final weeks of the first Trump administration that made it easier to fire federal employees who previously had civil service protections.

President Joseph R. Biden Jr. revoked Schedule F by executive order as one of his first official acts; Mr. Trump reinstated it on Inauguration Day this year.

Mr. Dans says his family moved to Charleston, S.C., where his wife is from, in the second half of the first Trump presidency; at the time she was pregnant with their fourth child. In early 2022, he joined the Heritage Foundation and started its 2025 Presidential Transition Project, best known as Project 2025.

The project’s goal was to seed the ground for a future Republican president by preparing lists of specific policy goals and vetted job candidates. Mr. Dans brought in contributors from more than 50 right-of-center organizations and within a year he put out the project’s 920-page policy book, “Mandate for Leadership.”

Despite its ambitious, far-reaching goals, the document largely remained out of the public consciousness until last year, when Democrats used it as a point of attack on Mr. Trump, then the presumptive Republican nominee for the presidency.

As Democrats increasingly focused on it, Mr. Trump — who had previously praised Project 2025 — publicly disowned it in July of last year, saying he had nothing to do with it and had “no idea who is behind it.”

Mr. LaCivita also publicly distanced the campaign from the initiative. “Anyone or any group trying to misrepresent their influence with President Trump and his campaign — it will not end well for you,” Mr. LaCivita said in a statement at the time.

By the end of that month, Mr. Dans was out at Heritage. That September, he told The New York Times that he had been “blindsided” by the response, and criticized the way Mr. LaCivita and Mr. Trump’s top adviser, Susie Wiles, were running the campaign. Heritage fired back with accusations that Mr. Dans had been pushed out over workplace behavior issues, but after Mr. Trump’s victory, Heritage walked back its comments, saying that the “previously reported concerns did not involve any issues with his integrity or inappropriate/offensive behavior towards women” and that the two sides had “agreed to resolve” their differences.

And despite myriad headlines about the falling-out between Mr. Dans and Heritage, he called any claims of a divisive split “fake news.”

Kevin D. Roberts, the Heritage Foundation president, did not respond to a message seeking comment about Mr. Dans’s campaign.

Mr. Dans has recently spent much of his time in South Carolina, and has repeatedly praised Mr. Trump’s policy agenda, calling his executive orders a kind of vindication, given how many of them have aligned with Project 2025.

“It’s beyond my wildest dreams,” he told Politico in March. In April, he wrote a wide-ranging essay in The Economist that cast himself as a staunch defender of Mr. Trump and his agenda and attacked federal judges and Democrats. It also has the feel of an early policy paper for a man with ambitions for public office.

“To make America run again, its government needs to be restored to one of, by and for the people, not a cadre of unelected, revolving-door bureaucrats,” Mr. Dans wrote. “How does this all play out? Will Mr. Trump rebuild an America First skyline or will the malaise of the status quo win out? My bet’s on Trump the builder.”