By Susan Cornwell and Yasmeen Abutaleb

WASHINGTON (Reuters) – U.S. Republicans cleared the first hurdle in their plan for the massive healthcare system overhaul backed by President Donald Trump, despite concerns among Democratic lawmakers, hospitals and insurers about its unknown costs and impact on coverage.

The House of Representatives Ways and Means Committee approved the bill along party lines on Thursday morning after debating the draft legislation for nearly 18 hours.

The chamber’s Energy and Commerce committee continued its own marathon session after Republican leaders earlier this week unveiled the plan, which would undo much of the 2010 Affordable Care Act, popularly known as Obamacare.

Republicans, who control the House and the Senate, are eyeing mid-April for passage of the bill.

“This is an historic step, an important step in the repeal of Obamacare,” said Republican Representative Kevin Brady, chairman of the House Ways and Means committee after it voted 23-16 for the measure.

The legislation would end the financial penalty for not owning health insurance, reverse most Obamacare taxes, introduce a smaller system of tax credits based on age rather than income, and overhaul Medicaid, the government health insurance program for the poor.

The American Hospital Association, the American Medical Association and other hospital groups have come out against the bill. The proposed changes to Medicaid have weighed on shares of hospitals, particularly Community Health Systems and Tenet Healthcare Corp, as investors worry about less government reimbursement.

Obamacare also enabled 20 million previously uninsured people to obtain coverage. About half came from a Medicaid expansion that the new bill would end.

America’s Health Insurance Plans, which represents Anthem Inc and other insurers, said tax credits for the individual insurance market did not go far enough.

The House Ways and Means committee, which was looking at the tax-related provisions of the bill, made no changes, despite dozens of attempts by Democrats to introduce amendments.

The fast-emerging disorder around the bill, Trump’s first legislative test, follows the chaos triggered by his travel ban on citizens from several Muslim-majority nations that was later revised.

Trump and fellow Republicans campaigned last year on a pledge to dismantle Obamacare, the signature domestic policy achievement of Democratic former president Barack Obama. They have called it government overreach that had ruined the more than $3 trillion U.S. healthcare system.

RESISTANCE

But Republican lawmakers face resistance from conservatives within their own ranks who say the bill, which would create a system of tax credits to coax people to buy private insurance on the open market, is not radical enough.

Health and Human Services Secretary Tom Price has tried to allay those concerns, saying the bill is part of a three-phase plan. But he told Fox News: “The message might not have been absolutely piercing to folks.”

In a series of tweets early on Thursday, Republican Senator Tom Cotton urged his House colleagues to pull back, saying their measure could not pass the Senate without major changes. “What matters in long run is better, more affordable health care for Americans, NOT House leaders’ arbitrary legislative calendar,” he wrote.

Representative Steve King said on CNN that his fellow Republicans must act now. “If nothing gets done here in this Congress, we are stuck with Obamacare,” he said.

Democrats denounce the bill as a gift to the rich and say informed debate on it is impossible without knowing its cost.

“The millionaires and billionaires, they’re going to do just great,” Democratic Senator Chris Van Hollen, a member of the Senate Budget Committee told MSNBC. But for working Americans, “this makes that squeeze much tighter and provides windfall tax breaks to the wealthy.”

House Democratic leader Nancy Pelosi cited the lack of analysis of the bill by the nonpartisan Congressional Budget Office. “This is decision-making without the facts,” she told reporters on Wednesday.

Republicans said they had asked the CBO to provide a preliminary estimate of the cost of the bill and expect to have that analysis by the time it hits the House floor.

“CONSTRUCTIVE IMPROVEMENTS”

Dan Holler, spokesman for the powerful conservative group Heritage Action, also sought more information. “Americans deserve full transparency, which includes the full budget score,” he said.

But some Republicans have cast doubt on the accuracy of CBO estimates, suggesting the agency’s initial assessment of the cost of Obamacare had proved far wide of the mark.

“If you’re looking at the CBO for accuracy, you’re looking in the wrong place,” White House spokesman Sean Spicer said on Wednesday.

Trump and Vice President Mike Pence met with leaders of conservative groups who have concerns about the bill on Wednesday. A White House official later said they were open to “constructive improvements.”

Once the two committees have approved their parts of the legislation, both will go to the House Budget Committee, which is expected to merge them into one bill that will then be voted on by the full chamber.

House Speaker Paul Ryan wants that vote to happen this month so the bill can move to the Senate for consideration.

Medicaid Chief Medical Officer Andrey Ostrovsky said on Twitter that he was aligned with experts who oppose the bill, breaking with the administration.

(Additional reporting by David Morgan, Brendan O’Brien, Susan Heavey and Doina Chiacu; Writing by Nick Tattersall; Editing by Lisa Von Ahn)