U.S. Senate Republicans plan to unveil the text of their draft health-care bill on Thursday as senators struggle over issues such as the future of the Medicaid program for the poor and bringing down insurance costs.

Republicans in the chamber have been working for weeks behind closed doors on legislation aimed at repealing and replacing major portions of the Affordable Care Act, former Democratic President Barack Obama's signature health-care law, popularly known as Obamacare.

The effort has been plagued from the start by tensions between moderates and conservatives, which surfaced again on Tuesday. Democrats have also criticized the behind-the-scenes meetings, staging a protest on the Senate floor on Monday.

"Republicans are writing their health-care bill under the cover of darkness because they are ashamed of it," Senate Democratic leader Chuck Schumer charged.

President Donald Trump campaigned on a promise to repeal Obamacare. The 2010 law extended insurance coverage to millions of Americans through both subsidized private insurance and an expansion of Medicaid.

The Republican-controlled House of Representatives narrowly approved its version of repeal last month.

Trump has urged the Republican-led Senate to pass a more "generous" bill than that approved by the House, whose version he privately called "mean," according to congressional sources.

An estimated 23 million people could lose their health care under the House plan, according to the non-partisan Congressional Budget Office. Senate Majority Leader Mitch McConnell said on Tuesday the Senate health-care bill would be different from the House version, but he did not elaborate.

In the Senate, moderates including Senator Shelley Moore Capito have argued for a long, seven-year phase-out to the Medicaid expansion that happened under Obamacare. But Senator John Thune, a member of the Republican leadership, said on Tuesday the phase-out in the bill might just be three years.

Capito said on Tuesday she was also concerned the Senate health-care plan might cap Medicaid spending and shift it to a lower growth rate in 2025. "That's an issue," she said.