-- Four Republican senators say "not ready to vote" for bill

-- Democrats attack Senate Republican bill as "even meaner"

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-- US hospital stocks surge after legislation unveiled

US Senate Republicans on Thursday 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 several senators voiced skepticism.

Four conservative lawmakers said they could not support it in its current form, leaving Republicans short of the votes they need for passage. Democrats are united in opposition.

The 142-page proposal, worked out in secret by a group led Senate Majority Leader Mitch McConnell, aims to deliver on a central campaign promise of President Donald Trump by rolling back former President Barack Obama's signature healthcare law, which has provided coverage to millions of Americans since it was passed in 2010.

Republicans view the law, formally known as the Affordable Care Act, as a costly government intrusion into the private marketplace.

Trump welcomed the bill but indicated that changes may be in store.

"I am very supportive of the Senate #Healthcarebill. Look forward to making it really special!" he wrote on Twitter.

Trump urged the House of Representatives to pass a similar bill in May, only to criticize it in private as "mean" once it passed. He said on Wednesday he wanted a health plan "with heart."

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," said Senate Democratic leader Chuck Schumer. "The Senate bill may be even meaner."

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," he wrote.

The Senate's most conservative members said the plan did not do enough to scale back the US government's role.

"This current bill does not repeal Obamacare. It does not keep our promises to the American people," said Senator Rand Paul, who along with fellow Republican Senators Ted Cruz, Mike Lee and Ron Johnson said they could not support it in its current form.

Shares of hospital companies and health insurers rose on the bill's release, with the overall S&P 500 healthcare sector closing up 1.1 percent at an all-time high.

"The initial proposal I think is more generous and more positive to the industry than expected," said Jeff Jonas, a portfolio manager with Gabelli Funds.

SHARPER CUTS TO MEDICAID

Over months of often bitter debate, Republicans have struggled to craft legislation that lowers costs and reduces government involvement, while minimizing the inevitable disruptions that would come with a revamp of a sector that accounts for one-sixth of the world's largest economy.

The nonpartisan Congressional Budget Office found that the House bill would kick 23 million Americans off their health plans, and the legislation is unpopular with the public. Fewer than one in 3 Americans supports it, according to Reuters/Ipsos polling.

The Senate measure maintains much of the structure of the House bill, but differs in several key ways.

The Senate bill would phase out Obamacare's expansion of the Medicaid program for the poor more gradually than the House version, waiting until after the next presidential election in 2020, but would enact deeper cuts starting in 2025. It would also allow states to add work requirements for some of the 70 million Americans who depend on the program.

The legislation also provides more generous tax subsidies than the House bill to help low-income people buy private insurance.

Those subsidies would be based on income, rather than the age-based subsidies contained in the House bill - a "major improvement," according to Republican Senator Susan Collins, a key moderate who has expressed concern over the bill's impact on the poor.

The Senate legislation provides less money, however, for the opioid epidemic, allocating $2 billion in 2018, compared with $45 billion over 10 years in the House version.

Both versions would repeal the 3.8 percent net investment income tax on high earners, a key target for Republicans.

They also would repeal a penalty imposed on large employers that do not provide insurance to their workers, and remove the fine that Obamacare imposes on those who choose to go uninsured.

Policy experts said that would keep more young, healthy people out of the market and likely create a sicker patient pool.

The Senate bill would provide money to stabilize the individual insurance market, allotting $15 billion a year in 2018 and 2019 and $10 billion a year in 2020 and 2021.

It proposes defunding Planned Parenthood for a year, but abortion-related restrictions are less stringent than the House version because of uncertainty over whether they would comply with Senate rules. They could be included in another Senate bill.

McConnell said Democrats chose not to help frame the bill, which Republicans say would fix a collapsing health marketplace.

"Republicans believe we have a responsibility to act, and we are," he said.

Democrats say they offered to help fix Obamacare but were rebuffed.

The bill's real-world impact is not yet known, but the CBO is expected to provide an estimate early next week.

As lawmakers spoke about the legislation on the Senate floor, a protest erupted outside McConnell's personal office, with many people in wheelchairs blocking a hallway, holding signs and chanting: "No cuts to Medicaid." US Capitol Police said 43 protesters were arrested and charged with obstruction.

Aside from the quartet of conservatives, none of the other 48 Republican senators appeared to reject the bill out of hand. But several said they would check with home-state constituents before taking a position.

"I expect there's going to be a number of changes between now and the final vote," said Senator John Barrasso of Wyoming.