The conservative Koch network is promising to spend millions of dollars to defeat the health care overhaul backed by President Donald Trump and top House Republicans.

The 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 comes 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. He added, "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 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, but we will also be there to support and thank the champions who stand strong and keep their promise," Americans for Prosperity president Tim Phillips said.