Two more Republican senators say they oppose their party's replacement for President Obama's health reforms, leaving leaders short of a majority, BBC reports.
Mike Lee and Jerry Moran both announced they could not support the bill in its current form.
Republicans hold 52 seats in the 100-member Senate, and with two other Republicans coming out against the bill they could not afford more defections.
President Trump made repealing Obamacare a key campaign pledge.
The two senators simultaneously announced their opposition to the planned reforms.
While saying there were "serious problems" with Obamacare, Mr Moran said "we should not put our stamp of approval on bad policy".
The plan retains key Obamacare taxes on the wealthy, while allowing insurers to offer less coverage and imposing sharp cuts to healthcare for the poor.
Congress is delaying its summer holiday in a bid to overturn former President Barack Obama's 2010 legislation.