A network of top Republican donors, led by the Koch brothers, are launching an 11th hour bid to sink the health care bill with a new fund for the 2018 midterms available only to members who vote "No".

Americans for Prosperity and Freedom Partners announced the "seven-figure fund" on Wednesday night. They oppose the bill because it falls short of a full Obamacare repeal.

The fund could complicate White House efforts to win over conservative holdouts.