WASHINGTON — A prominent Washington lobbyist close to Kevin McCarthy, the House minority leader, is warning Republican political consultants that they must choose between working for Representative Liz Cheney or Mr. McCarthy, an ultimatum that marks the full rupture between the two House Republicans.

Jeff Miller, the lobbyist and a confidant of Mr. McCarthy’s dating to their youthful days in California politics, has conveyed this us-or-her message to Republican strategists in recent weeks, prompting one fund-raising firm to disassociate itself from Ms. Cheney, a Republican from Wyoming.

In response, The Morning Group, a fund-raising firm she hired to help prepare for a primary next year against a challenger endorsed by former President Donald J. Trump, informed her last month they could no longer work on her campaign, according to Republicans familiar with the matter.

Mr. Miller’s warnings illustrate the disintegration of the relationship between the two lawmakers, who began this year serving together in the House Republican leadership. They also underline Mr. McCarthy’s willingness to wield his leadership position to undercut Ms. Cheney’s re-election and head off an impediment to his claiming the speakership, should Republicans win a House majority next year. Were Ms. Cheney to return to Congress, she would loom as a potential instigator of any effort to block Mr. McCarthy from leading their party in the House.

After initially defending Ms. Cheney to House Republicans angry at her for voting to impeach Mr. Trump earlier this year, Mr. McCarthy abandoned her after she continued to speak out against the former president. In May, with Mr. McCarthy’s blessing, party lawmakers ousted Ms. Cheney from her role as the third-ranking House Republican.

Since then, Ms. Cheney has sharply criticized Mr. McCarthy and has made clear she will not support him as leader or, if Republicans take the House next year, as speaker. After she joined the House select committee investigating the Jan. 6 attack, organized by Speaker Nancy Pelosi, Mr. McCarthy called Ms. Cheney and another dissident on the panel “Pelosi Republicans.”

The Republican leader’s allies believe Ms. Cheney, by continuing to lament Mr. Trump’s grip on the party, is threatening their prospects in the midterm elections at a moment when President Biden’s declining approval ratings have otherwise left the G.O.P. well positioned.

In an interview, Mr. Miller, who became close to Mr. Trump during his administration and raised more than $100 million for his re-election efforts, said Republican consultants were on notice.

“She’s not just undermining Kevin but the whole G.O.P. conference,” Mr. Miller said of Ms. Cheney. “You’re either with Kevin, and the conference, or the person undermining them. You can’t serve two masters.”

The lobbyist declined to discuss his communications with party strategists or to say if Mr. McCarthy had prompted him to issue the ultimatum. Mr. Miller, whose clients include Amazon and the Pharmaceutical Research and Manufacturers of America, does not work for the leader and is not paid by him. However, they are the closest of friends, and have been since they worked together for former Representative Bill Thomas, the powerful Bakersfield Republican whom Mr. McCarthy succeeded in Congress.

A spokesman for Mr. McCarthy declined to comment.

In Washington’s Republican circles, the relationship between lobbyist and leader is well known and Mr. Miller’s word is authoritative, as made clear by the fund-raising firm’s cutting ties with Ms. Cheney.

The National Republican Congressional Committee, the House G.O.P.’s campaign arm, does not get involved in primaries.

But the Congressional Leadership Fund, the main House Republican “super PAC,” which Mr. McCarthy effectively controls, is not supporting Ms. Cheney, though it is supporting some of the other House Republicans who voted to impeach Mr. Trump, according to a Republican official familiar with the group’s strategy.

To Ms. Cheney, the behind-the-scenes campaign against her only highlights how beholden to Mr. Trump Mr. McCarthy is — and the lengths to which he will go as he grasps for the speakership.

A spokesman for Ms. Cheney, Jeremy Adler, blistered Mr. McCarthy for working against her candidacy while supporting some of the most extreme members of the House Republican conference.

“It’s sad but not surprising that Kevin McCarthy is continuing down the morally bankrupt path of embracing House Republicans who are white supremacists and conspiracy theorists, but attacking Liz Cheney for telling the truth and standing for the Constitution,” Mr. Adler said.

A principal at The Morning Group, Mackenzie Jortner Dolan, declined to comment on the firm’s decision to drop Ms. Cheney. As of mid-September, Ms. Dolan was still working for the congresswoman, emailing Republican donors to promote a fund-raiser Ms. Cheney held in Dallas on Monday with former President George W. Bush.

She raised nearly $400,000 at the Bush event, for which tickets began at $1,000.

Ms. Cheney has not suffered financially for her outspoken criticism of Mr. Trump and Mr. McCarthy. Through the end of September, she had raised over $5 million and had more than $3.6 million on hand.

She has retained support from an array of leading Republicans, including Mr. Bush; Mitch McConnell, the Senate minority leader; and the former House Speakers John Boehner and Paul Ryan.

She has, however, emerged as perhaps Mr. Trump’s top priority to defeat. After meeting with a series of would-be challengers, Mr. Trump in September endorsed Harriet Hageman, a Cheyenne attorney who had taken part in a failed, last-ditch effort to strip him of the 2016 Republican presidential nomination two months after he had clinched it.

Ms. Hageman, who was once a Cheney family friend and served as an adviser to Ms. Cheney’s short-lived 2014 Senate campaign, raised just over $300,000 for the quarter, but she only filed her candidacy on Sept 9.

Mr. Trump has endorsed a handful of other Republicans challenging the lawmakers who voted to impeach him and was exultant last month when Representative Anthony Gonzalez of Ohio, who cast one of the 10 Republican votes to impeach, said he would retire from Congress rather than run against a former Trump White House official.

“1 Down, 9 To Go!” the former president said in a statement at the time.