David Koch, executive vice president of Koch Industries, applauds during an Economic Club of New York event in New York. Reuters/Brendan McDermid Billionaire conservative donors Charles and David Koch are launching a last-ditch effort to sink 'Trumpcare', enticing Republicans in the House with millions in campaign funding — if they vote against the bill.

The House of Representatives is scheduled to vote on the American Health Care Act, as the bill is formally titled, on Thursday.

It's President Donald Trump's first major piece of legislation, and would deliver on Republicans' seven-year promise to "repeal and replace" the Affordable Care Act, the healthcare law better known as Obamacare.

But all House Democrats, and roughly 29 Republicans have said they will vote "no," which wouldn't give the AHCA enough votes to pass the chamber.

The Koch brothers are trying to sweeten the deal for House Republicans to go against the president and the Speaker of the House, Paul Ryan. The Koch network's leading organizations, Americans for Prosperity and Freedom Partners, announced late Wednesday the creation of a special fund to support House members who vote against the health care bill.

The announcement, which came on the eve of the House vote, marks the influential conservative network's most aggressive move against the health care proposal, which is under attack from the right and the left.

"The bill as it stands today is Obamacare 2.0," said James Davis, executive vice president of Freedom Partners, in a statement announcing the new fund. "We will stand with lawmakers who keep their promise and oppose this legislation — and work toward a solution that reduces costs and provides Americans with the relief they need and deserve."

The organizations backed by billionaire industrialists Charles and David Koch declared their formal opposition to the GOP health care proposal on Monday.

Like other conservatives, they decried the legislation as a new entitlement program that doesn't go far enough in repealing President Barack Obama's health care law. Moderate Republicans, meanwhile, are concerned by projections of 24 million Americans losing coverage in a decade and higher out-of-pocket costs for many low-income and older people.

The Koch-backed fund is being described only as "seven figures" and will go toward political ads, direct mail, and other grassroots activity.

"We have a history of following up and holding politicians accountable," Americans for Prosperity president Tim Phillips said, "but we will also be there to support and thank the champions who stand strong and keep their promise."

Given the upheaval, Republicans have delayed a meeting of their House members originally scheduled for 9 a.m. ET Thursday until after negotiations between GOP holdouts in the bill and the White House. It remains unclear whether the bill will pass.