Senate Minority leader Chuck Schumer stands during a press conference after Senate Republicans unveiled their version of legislation that would replace Obamacare on Capitol Hill in Washington, US, June 22, 2017. [Photo/Agencies]

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 "withheart."

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, MikeLee and Ron Johnson said they could not support it in it scurrent 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 morepositive to the industry than expected," said Jeff Jonas, aportfolio manager with Gabelli Funds.