Two Democratic senators threatening to derail Joe Biden’s agenda have been condemned by anti-corruption watchdogs for accepting a flood of money from Republican and corporate donors.

Joe Manchin and Kyrsten Sinema present the last hurdle to the US president’s social spending and climate package after it was passed by Democrats in the House of Representatives earlier this month.

Having already pressured colleagues to cut the cost of the Build Back Better plan in half, the conservative duo continue to raise concerns about its $1.75tn price tag and sprawling ambition.

Manchin and Sinema’s status as holdouts in a Senate evenly divided between 50 Democrats and 50 Republicans has guaranteed outsized influence and attention. It also appears to be lucrative.

In September, Sinema received a cheque from Stanley Hubbard, a billionaire Republican donor who is considering a similar contribution to Manchin because of their work to reduce the bill’s price tag, the New York Times reported this week. “Those are two good people – Manchin and Sinema – and I think we need more of those in the Democratic party,” Hubbard was quoted as saying.

The newspaper also revealed Manchin, of West Virginia, and Sinema, of Arizona, travelled to an $18m mansion in Dallas for a summer fundraiser attended by Republican and big business donors who have praised their efforts to pare down the Build Back Better bill.

Manchin, for example, opposed popular provisions such as paid family leave and a clean electricity programme that would boost wind and solar power while phasing out coal and gas, while Sinema rejected an increase of personal and corporate income tax rates. Their stances have not gone unnoticed by Wall Street and wealthy conservatives.

The $3.3m raised by Manchin’s campaign in the first nine months of this year was more than 14 times his haul at the equivalent stage last year, the New York Times added, while the $2.6m taken in by Sinema’s campaign was two and a half times what she netted over the same period in 2020.

Such sums, which include contributions from political action committees and donors linked to the finance and pharmaceutical industries, have raised ethical concerns over whether Democrats Manchin and Sinema are being unduly influenced.

Kyle Herrig, president of Accountable.US, a non-partisan watchdog that targets government corruption, said: “What else but industry money could explain the manufactured excuses for resisting Build Back Better considering it remains extremely popular, is fully paid for, and would cut costs and taxes for most everyday people in Arizona and West Virginia?

“Corporate interests and billionaires have done very well even during the pandemic and don’t need more special treatment. Senators Sinema and Manchin have a chance they may not get again to help so many regular families and seniors get ahead for a change, so why squander it over complaints from a handful of rich interests that exploit tax loopholes and ship jobs overseas?”

Accountable.US said its own tracking of corporate activity found that Manchin had taken more than $1.5m and Sinema nearly $1m from corporate interests opposed to the Build Back Better plan as of September. It added that Sinema abandoned prior support for lowering prescription drug prices after a deluge of money from the pharmaceutical industry in the third quarter.

Such patterns have angered grassroots activists who say Sinema’s positions are out of sync with the stated needs and views of her own constituents in Arizona.

Stephany Spaulding, a spokesperson for Just Democracy, a coalition of more than 40 civil rights and social justice groups, said in response to the New York Times article: “Senator Sinema’s approval ratings among her base have plummeted in recent months and this donor report shows why. She’s busy chasing out-of-state corporate dollars instead of fighting for her constituents’ needs.

“Crucial policies like infrastructure, Build Back Better, Medicaid expansion and voting rights are all incredibly popular, but once again she’s demonstrated how frail her commitment to Black and brown Arizonans is.”

The House approved the Build Back Better legislation 220-213 as every Democrat but one backed it, overcoming unanimous Republican opposition. After a brief Thanksgiving hiatus it heads to the Senate, where changes are certain as moderates and progressives wrangle over its cost and scope.

Chuck Schumer, the Senate majority leader, told reporters on Sunday: “The House did a very strong bill. Everyone knows that Manchin and Sinema have their concerns, but we’re going to try to negotiate with them and get a very strong, bold bill out of the Senate which will then go back to the House and pass.”

Manchin is facing pressure to support a provision that would grant four weeks of paid family and medical leave, bringing America into alignment with most western industrial democracies. Sinema’s priorities and red lines have been harder to discern, a source of frustration in its own right.

Some observers were unsurprised that the senators are attracting cash from the right. Jordan Libowitz, communications director of Citizens for Responsibility and Ethics in Washington (Crew), said: “It’s not unusual for megadonors to occasionally cross party lines with their donations. For some, this is hedging their bets, for others, rewarding a politician who came through on an issue they care about.

“Ultimately, money follows power. Manchin and Sinema look to be the deciding votes on major legislation, so it really isn’t surprising that people from the other side of the aisle will be trying to win influence with them.”

But others described it as an indictment of the influence of money in politics. Larry Jacobs, director of the Center for the Study of Politics and Governance at the University of Minnesota, said: “It’s an outrage. Candidates are in such a hunger for amassing large campaign war chests that they’re going to provide privileged access to the very interests are supposed to be regulating.”

He added: “The appearance of what’s going on here – lavish public fundraising events – is a dagger through the trust and legitimacy of American democracy. It just captures the worst fears that Americans have that politicians are up for sale. And I think that public perception is toxic.”

Manchin and Sinema’s offices did not respond to requests for comment.