US President Donald Trump is pressurising divided Senate Republicans to pass their stalled health care bill in the next few weeks.
But the bill has come under renewed attack from within the party, with a moderate senator calling for a bipartisan approach and a conservative saying Republicans were "at impasse" and that leaders were trying to "buy off" votes.
Senators were returning to Washington after a week-long July 4 recess that saw Republican support erode for a bill fashioned by Senate majority leader Mitch McConnell.
The measure would erase much of former president Barack Obama's healthcare law and scale back its coverage requirements, end its penalty on people not buying coverage, cut payments for medical services for low-income citizens and eliminate tax boosts on wealthier people.
"I cannot imagine that Congress would dare to leave Washington without a beautiful new HealthCare bill fully approved and ready to go!" Mr Trump tweeted.
Mr McConnell was studying changes aimed at gaining Republican support for the bill, which has encountered opposition from both ends of the party's spectrum. Republican leaders hoped they could push it through the Senate before Congress departs again for an August recess.
Proposed revisions include rolling back some of its Medicaid cuts, adding additional money to combat abuse of opioids and other drugs, beefing up healthcare subsidies for lower earners and allowing insurers to sell low-cost policies offering meagre coverage.
Moderate senator Susan Collins of Maine told reporters she hoped Mr McConnell's changes "are more than tweaks and they're an overhaul of the approach, or else I won't be able to support it".
She said politicians of both parties "should work together to fix" flaws in Mr Obama's law - an approach that would contradict Republican doctrine, which has long called for repealing the 2010 statute.
Ms Collins has criticised the bill for causing millions of people to lose healthcare coverage and blocking federal payments to Planned Parenthood.
Conservative senator Rand Paul, who has repeatedly said the Republican bill does not go far enough in dismantling Mr Obama's statute, said it will "have to look more like repeal" for him to support it.
"I think we're still at impasse," Mr Paul told reporters on Monday. "Some people think that they can add enough goodies, federal spending on there, to buy off the votes of Republicans."
Mr Paul said he spoke with Mr Trump over the weekend. He said he thinks Mr Trump wants "what I want, which is a conservative vision of healthcare which involves repealing Obamacare".
At least a dozen Republican senators have expressed opposition to Mr McConnell's initial bill or criticised it. Republicans hold a 52-48 majority and Democrats stand united against the bill, meaning that just three Republican defections will defeat it.
Mr McConnell has acknowledged that if the broader effort fails, he would want the Senate to turn to a smaller bill aimed at shoring up insurance marketplaces where companies have stopped selling policies and premiums are soaring.
But Mr Trump and some conservatives - including Mr Paul - have said if Mr McConnell's wide-ranging bill fails, they would favour an initial bill repealing Mr Obama's law, followed by a second measure somehow replacing it.
That sequence - Mr Trump and party leaders favoured it early this year but then abandoned it - would face all but certain defeat. Many Republicans remain unwilling to scuttle Mr Obama's statute without enactment of a replacement.
Conservative senator Ted Cruz has proposed letting insurers sell any policies they would like, as long as they also sell one that covers a list of services like maternity care that Mr Obama's law requires. Its fate was uncertain.
The plan has drawn support from the White House and some congressional conservatives. But it has limited appeal to Republican moderates who worry it would lead to unaffordable prices for people with pre-existing medical conditions. That's because younger, healthy customers would be unlikely to buy extensive policies, leaving mainly those with costly medical problems purchasing them.
Meanwhile, a study found that the number of uninsured adults has grown by two million this year, underscoring that recent coverage gains have begun to erode.
The Gallup-Sharecare Well-Being Index, published on Monday, found that the uninsured rate among adults was 11.7% in the second three months of this year, compared with a record low of 10.9% at the end of last year.
The losses were concentrated among younger adults and people buying their own health insurance policies, the survey found.
The reduction could reflect rising premiums and dwindling choices in the insurance markets created under Mr Obama. It could also flow from Trump administration actions and comments about withholding support for the law, which some insurers have said are making them reluctant to offer coverage in some areas.