By Amanda Becker and Yasmeen Abutaleb

WASHINGTON (Reuters) – U.S. Senate Republicans begin their final push on Thursday to unravel Obamacare, seeking to wrap up their seven-year offensive against former Democratic President Barack Obama’s signature healthcare law that extended insurance coverage to millions.

Republicans leaders hope a pared-down “skinny” bill that repeals several key Obamacare provisions can gain enough support to pass after several attempts at broader legislation failed to win approval earlier this week.

The skinny bill’s details will be released at some point on Thursday, before the Senate embarks on a marathon voting session that could extend into Friday morning. The legislation is expected to eliminate mandates requiring individuals and employers to obtain or provide health insurance, and abolish a tax on medical device manufacturers.

The effort comes after a chaotic two-month push by Senate Republicans to pass their version of legislation that made it out of the Republican-controlled House of Representatives in May.

Members of the party, including President Donald Trump, campaigned on a pledge to repeal and replace what they say is a failing law that allows the government to intrude in people’s healthcare decisions.

Republicans were optimistic about the skinny bill’s chances of receiving at least 50 votes in the Senate where they hold a 52-48 majority.

Senator John Cornyn, the chamber’s No. 2 Republican, said the bill, once approved, would go to a special negotiating committee of lawmakers from both chambers that would reconcile the House and Senate versions into a single piece of legislation.

Republican leaders had tapped a group to craft legislation largely behind closed doors, exposing rifts within the party. While conservatives said the group’s proposals did not go far enough, moderates said they could not support measures estimated to deprive tens of millions of health insurance.

The Senate voted 55-45 on Wednesday against a simple repeal of Obamacare, which would have provided a two-year delay so Congress could work out a replacement. Seven Republicans opposed the bill. On Tuesday, senators rejected the repeal-and-replace plan Republicans had been working on since May.

Senate Majority Leader Mitch McConnell can lose only two Republican votes to pass healthcare legislation. Even then, he would have to call on Vice President Mike Pence to cast a tie-breaking vote as head of the Senate. Democrats are united in opposition.

GOVERNORS SEEK INVOLVEMENT

A bipartisan group of 10 governors urged senators in a letter on Wednesday to start over and use a drafting process that includes governors from both parties. Governors of Nevada, Ohio, Louisiana, Pennsylvania and Colorado were among those who signed the letter, all of whose states have Republican senators.

The Congressional Budget Office, a nonpartisan research agency, estimated on Wednesday that a combination of provisions that might go into the skinny bill would lead to 16 million people losing their health coverage by 2026.

It had earlier estimated that the two other bills rejected by the Senate this week would have led to 22 million to 32 million people losing their health insurance by 2026.

Senate Democratic leader Chuck Schumer criticized Republican leaders for crafting a “yet-to-be-disclosed final bill” in secret.

“We don’t know if skinny repeal is going to be their final bill, but if it is, the CBO says it would cause costs to go up, and millions to lose insurance,” Schumer said on the Senate floor.

(Additional reporting by Susan Cornwell, Richard Cowan and David Morgan; Writing by Lisa Lambert; Editing by Peter Cooney)