G.O.P.

WASHINGTON — Over the summer, as he was working to scale back President Biden’s domestic agenda, Senator Joe Manchin III of West Virginia traveled to an $18 million mansion in Dallas for a fund-raiser that attracted Republican and corporate donors who have cheered on his efforts.

In September, Senator Kyrsten Sinema of Arizona, who along with Mr. Manchin has been a major impediment to the White House’s efforts to pass its package of social and climate policy, stopped by the same home to raise money from a similar cast of donors for her campaign coffers.

Even as Ms. Sinema and Mr. Manchin, both Democrats, have drawn fire from the left for their efforts to shrink and reshape Mr. Biden’s proposals, they have won growing financial support from conservative-leaning donors and business executives in a striking display of how party affiliation can prove secondary to special interests and ideological motivations when the stakes are high enough.

Ms. Sinema is winning more financial backing from Wall Street and constituencies on the right in large part for her opposition to raising personal and corporate income tax rates. Mr. Manchin has attracted new Republican-leaning donors as he has fought against much of his own party to scale back the size of Mr. Biden’s legislation and limit new social welfare components.

It is not unusual for well-heeled political activists and business interests to spread a smattering of cash across party lines. Representative Liz Cheney, Republican of Wyoming, collected a handful of checks from major Democratic donors this year as she bucked her party leadership’s defense of former President Donald J. Trump.

But the stream of cash to the campaigns of Ms. Sinema and Mr. Manchin from outside normal Democratic channels stands out because many of the donors have little history with them. The financial support is also notable for how closely tied it has been to their power over a single piece of legislation, the fate of which continues to rest largely with the two senators because their party cannot afford to lose either of their votes in the evenly divided Senate.

Their influence has been profound. The domestic policy bill, which would expand the social safety net and efforts to fight climate change, started out at $3.5 trillion and has been shrunk — mainly at the insistence of Mr. Manchin — to around $2 trillion; it could get smaller as the Senate takes up the version passed on Friday by the House. New spending measures were originally to have been paid for mostly through tax-rate increases on the wealthy and corporations — a component of the plan that had to be substantially rewritten because of Ms. Sinema’s opposition.

This month, the billionaire Wall Street investor Kenneth G. Langone, a longtime Republican megadonor who has not previously contributed to Mr. Manchin, effusively praised him for showing “guts and courage” and vowed to throw “one of the biggest fund-raisers I’ve ever had for him.”

In a statement to The New York Times, Mr. Langone, who has given an overwhelming majority of his millions of dollars in federal political donations to Republicans, said, “My political contributions have always been in support of candidates who are willing to stand tall on principle, even when that means defying their own party or the press.”

Stanley S. Hubbard, a billionaire Republican donor, wrote his first check to Ms. Sinema in September and said that he was considering doing the same for Mr. Manchin because of their efforts to trim the sails of the Democrats’ agenda. “Those are two good people — Manchin and Sinema — and I think we need more of those in the Democratic Party,” he said.

Cash has also poured in for Mr. Manchin and Ms. Sinema from political action committees and donors linked to the finance and pharmaceutical industries, which opposed proposals initially included in the domestic policy bill that the lawmakers helped scale back, including changes to Medicare and the tax-rate increases.

John LaBombard, a spokesman for Ms. Sinema, rejected any suggestion that campaign cash factored into her approach to policymaking. She was a lead negotiator on the bipartisan infrastructure deal that Mr. Biden signed last week, and during her time in the Senate, she has positioned herself as an ideologically flexible centrist willing to buck her party in representing a purple state.

“Senator Sinema makes decisions based on one consideration: what’s best for Arizona,” Mr. LaBombard said.

Mr. Manchin’s office did not respond to requests for comment. But he has long expressed concern that the legislation, if not pared back to the level he is seeking, would add to the budget deficit and could fuel inflation.

The lawmakers share a campaign finance consultant, who helped organize fund-raising swings through Texas for both lawmakers that yielded cash from Republican donors, as well as a fund-raiser for Ms. Sinema in Washington in late September with business lobbying groups that oppose the domestic policy bill.

Nelson Peltz, a billionaire investor who brought a Republican-heavy group of chief executives to have lunch with Mr. Manchin in Washington a few months ago, said the senator “understands that you can’t spend, spend, spend and feel there’s no recourse for it.”

Mr. Peltz, who donated to Mr. Manchin in 2017, has not given to Ms. Sinema, but he said that she had requested a meeting, which will take place in a few weeks.

Individual donors like Mr. Peltz, who over the years has donated nearly three times as much to Republicans as he has to Democrats at the federal level, offer the two Democratic senators a way to restock their campaign coffers — both are up for re-election in 2024 — at a time when they are unlikely to get an enthusiastic reception from some more traditional Democratic donors.

Mr. Manchin has long been to the right of his party on litmus-test issues like abortion rights and fossil fuels, while Ms. Sinema started her political career as a liberal activist before shifting to the center. One Wall Street executive joked that in his industry, Ms. Sinema — who as a young politician once likened political donations to “bribery” — was now referred to as “Saint Sinema” for opposing most of Mr. Biden’s proposed taxes on the wealthy. (She has, however, supported a 15 percent corporate minimum tax and other revenue-raising measures that will help pay for Mr. Biden’s legislative spending.)

Progressives are less amused and have accused both senators of undermining their party’s agenda at the behest of special interests.

Wealthy liberals recently began an effort to lay the groundwork for a primary challenge to Ms. Sinema in 2024, and the liberal group Demand Progress wrote in a petition that “a small group of right-wing Democrats backed by corporate cash, including Joe Manchin and Kyrsten Sinema, are trying to destroy” Mr. Biden’s legislative agenda.

This year, Mr. Manchin and Ms. Sinema have received donations from major Republican donors who had never before given to them, including James A. Haslam III, who owns the Cleveland Browns football team, and the Dallas real estate developer Harlan Crow, who is close to Justice Clarence Thomas of the Supreme Court.

Several other prominent Republican donors who supported Mr. Trump also wrote their first-ever checks to Mr. Manchin in the last few months. They include the Oklahoma oil and gas billionaire Harold Hamm, who pushed the former president to deregulate the energy industry; the Dallas-based lobbyist and investor Roy W. Bailey, who helped lead fund-raising for Mr. Trump’s inauguration and a pro-Trump nonprofit group; and the banker Andrew Beal, who donated a total of $3 million to a super PAC supporting Mr. Trump from 2018 through last year.

Executives at Goldman Sachs, including the firm’s president, John Waldron, combined to donate tens of thousands of dollars to Ms. Sinema in the spring and summer. In July, she attended a meet-and-greet at the offices of the Blackstone Group, which is headed by a major Republican donor; some Blackstone employees made donations around the same time. A handful of employees from the investment firm Apollo Global Management, including Marc J. Rowan, the chief executive and a major donor to predominantly Republican candidates and causes, donated to Ms. Sinema in late September after the firm sent a plea to industry contacts seeking donations for her.

G. Brint Ryan, the Republican donor who hosted the fund-raisers in Dallas for Mr. Manchin and Ms. Sinema, said the senators were “out of step with their party, but I tend to believe that they’re in the right.”

Mr. Ryan had not previously donated to Ms. Sinema and had not held fund-raisers for either before this year, though he donated $1,000 to Mr. Manchin’s 2018 re-election campaign.

The website for Mr. Ryan’s tax consulting firm says it works at “liberating our clients from the burden of being overtaxed.”

The firm’s lobbyists have been monitoring the debate in Congress over the tax implications of the domestic policy bill, according to disclosure filings. Mr. Ryan, who said in an email that the measure would “make a bad tax code worse and kill economic growth,” has ties to Republicans who have helped lead opposition to it.

He advised Mr. Trump on tax policy during his presidential campaign in 2016. One of the partners in Mr. Ryan’s tax consulting firm is Jeff Miller, a corporate lobbyist and close political adviser to Representative Kevin McCarthy of California, the House Republican leader.

Mr. Miller, who is a top Republican fund-raiser, helped steer Mr. Ryan’s team to people who could assist in planning the fund-raisers for Ms. Sinema and Mr. Manchin. And Mr. Miller’s wife gave to Ms. Sinema’s campaign.

In the days around the fund-raisers at his home, Mr. Ryan, his employees, his company’s political action committee and a relative’s law firm combined to donate nearly $80,000 to Ms. Sinema’s campaign and more than $115,000 to Mr. Manchin’s.

The $2.6 million raised by Ms. Sinema’s campaign through the first nine months of this year was two and a half times as much as she raised in the same period last year, while the $3.3 million raised by Mr. Manchin’s campaign was more than 14 times as much as his haul through the end of September last year.

Overall, Ms. Sinema’s campaign took in about $6.1 million in donations between the beginning of 2019 and the end of September, and it had $4.5 million in the bank with three years to go until she faces the voters in Arizona. Mr. Manchin’s campaign raised about $3.8 million and had $5.4 million on hand.

Kenneth P. Vogel reported from Washington, and Kate Kelly from New York. Jonathan Weisman contributed reporting.