US Senate Republicans have unveiled legislation that would replace Obamacare with a plan that scales back aid to the poor and kills a tax on the wealthy, but the bill's fate was quickly thrown into question as some senators voiced scepticism.
Four lawmakers - Rand Paul, Ted Cruz, Mike Lee and Ron Johnson - said on Thursday the bill did not go far enough and they could not support it in its present form, leaving Republicans short of the votes they need for passage.
The proposal, worked out in secret by a group led Senate Majority Leader Mitch McConnell, aims to deliver a campaign promise of President Donald Trump by rolling back former president Barack Obama's healthcare law, which has provided coverage to millions of Americans since it was passed in 2010.
Republicans view the law as a costly government intrusion into the private marketplace.
Trump welcomed the bill but indicated changes might be in store, tweeting: "I am very supportive of the Senate #Healthcarebill. Look forward to making it really special!"
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Trump urged the House of Representatives to pass a similar bill in May, only to criticise it in private as "mean" once it passed.
The Senate bill proposes repealing the 3.8 per cent net investment income tax on high earners retroactively to the start of 2017.
The tax was imposed to help pay for Obamacare.
The Senate bill maintains much of the structure of the House bill but differs in key ways.
It would phase out Obamacare's expansion of the Medicaid program for the poor and disabled over three years, from 2021 to 2024, and then enact deeper cuts in the program than the House version, beginning in 2025.
The legislation also reshapes subsidies to low-income people for private insurance.
Democrats immediately attacked the legislation as a callous giveaway to the rich that would leave millions without coverage.
"The president said the House bill was mean. The Senate bill may be even meaner," Senate Democratic leader Chuck Schumer said.
Obama weighed in on Facebook: "If there's a chance you might get sick, get old, or start a family - this bill will do you harm."
The nonpartisan Congressional Budget Office found the House bill would kick 23 million Americans off their health plans, and the legislation is unpopular with the public.
Less than one-third of Americans support it, according to Reuters/Ipsos polling.
