Senate Republicans have released their long-awaited bill to dismantle much of Barack Obama's healthcare law but ran into trouble as four GOP senators said they opposed it.
The bill would provide less-generous tax credits to help people buy insurance and let states get waivers to ignore some coverage standards that Obamacare requires of insurers.
It would also end the tax penalties under Mr Obama's law on people who do not buy insurance - the so-called individual mandate - and on larger firms that do not offer coverage to their employees.
The measure represents the Senate GOP's effort to achieve a top-tier priority for President Donald Trump and virtually all Republican members of Congress.
Senate majority leader Mitch McConnell hopes to push it through his chamber next week but solid Democratic opposition - and complaints from at least six Republicans - have left its fate unclear.
"We have to act," Mr McConnell said on the Senate floor. "Because Obamacare is a direct attack on the middle class and American families deserve better than its failing status quo."
Some Republican senators, as well as all the Senate's Democrats, have complained about Mr McConnell's proposal, the secrecy with which he drafted it and the speed with which he would like to move it to passage.
Mr McConnell has only a thin margin of error: The bill would fail if just three of the Senate's 52 GOP senators oppose it.
Democrats gathered on the Senate floor and defended Mr Obama's 2010 overhaul.
They said GOP characterisations of the law as failing are wrong and claimed the Republican plan would boot millions off coverage and leave others facing higher out-of-pocket costs.