US president Donald Trump has urged Senate Republicans not to "let the American people down" as the contentious debate over overhauling America's health care shifts to Congress' upper chamber.
Some senators have already voiced displeasure with the health care bill that cleared the House of Representatives last week, with Republicans providing all the Yes votes in the 217-213 count.
They cited concerns about potential higher costs for older people and those with pre-existing conditions, along with cuts to Medicaid.
Senator Susan Collins of Maine, a moderate Republican whose vote will be critical to getting a bill to Mr Trump's desk and who voiced similar concerns, said the Senate would not take up the House bill.
"The Senate is starting from scratch. We're going to draft our bill and I'm convinced we will take the time to do it right," she said.
Mick Mulvaney, Mr Trump's budget director, also said the version that gets to the president would probably differ from the House measure.
Such a scenario would then force the House and Senate to work together to forge a compromise bill that both houses can support.
Ms Collins also complained that the House rushed a vote before the Congressional Budget Office could complete its cost-benefit analysis.
Mr Trump sought to pressure Senate Republicans on the issue on sunday.
"Republican senators will not let the American people down!" he tweeted from his private golf course in central New Jersey, where he has stayed since Thursday.
"ObamaCare premiums and deductibles are way up - it was a lie and it is dead!"
Mr Trump has said the current system is failing as insurers pull out of markets, forcing costs and deductibles to rise.
Republican Senators will not let the American people down! ObamaCare premiums and deductibles are way up - it was a lie and it is dead!— Donald J. Trump (@realDonaldTrump) May 7, 2017
The White House scoffed at Democratic claims that voters would punish the Grand Old Party in the 2018 elections for upending former president Barack Obama's law.
"I think that the Republican Party will be rewarded," said Reince Priebus, Mr Trump's chief of staff.
House of Representatives Democratic leader Nancy Pelosi has threatened that GOP members will "glow in the dark" over their vote.
The House bill would end the health care law's fines on people who do not buy policies and erase its taxes on health industry businesses and higher earners.
It would dilute consumer-friendly insurance coverage requirements, like prohibiting higher premiums for customers with pre-existing medical conditions and watering down the subsidies that help people afford health insurance.
Mr Obama defended his signature achievement in Boston on Sunday night while accepting the John F Kennedy Profile in Courage Award.
"I hope that current members of Congress recall that it actually doesn't take a lot of courage to aid those who are already powerful, already comfortable, already influential," he said.
"But it does require some courage to champion the vulnerable, and the sick and the infirm."
Major medical and other groups, including the American Medical Association, opposed the House bill.
Democrats are also refusing to participate in any effort to dismantle Mr Obama's law, while some Republican senators object to cutting Medicaid, the federal-state health care programme for the poor and disabled.
The Affordable Care Act expanded Medicaid with extra payments to 31 states to cover more people but the House bill halts the expansion, in addition to cutting government spending on the programme, which Mr Trump's health chief argues is flawed and dictates too much from Washington.
Health and human services secretary Tom Price said states would get more freedom to experiment with the programme and make sure that people who rely on Medicaid received the care and coverage they needed.
"There are no cuts to the Medicaid programme," Mr Price insisted on Sunday, adding that resources were being doled out to allow states greater flexibility.
Ohio governor John Kasich questioned what would happen to the mentally ill, drug addicts and people with chronic illnesses under the changes proposed for Medicaid.
"They are going to be living in the emergency rooms again", potentially driving up health care costs, he said.
Senate majority leader Mitch McConnell plans to move forward under special procedures that allow legislation to pass with a simple majority vote, instead of the 60 usually required for major bills in the Senate.
That means McConnell can afford to lose just two senators - US vice president Mike Pence would vote to break a 50-50 tie in his constitutional role as vice president of the Senate.
House speaker Paul Ryan appeared resigned to the legislative reality that the bill he unveiled with great fanfare would be altered as part of a "multistage process".
"We think we need to do even more support for people who are older and also more support for people with pre-existing conditions," he said.
"The Senate will complete the job."