Eight to 10 Republican U.S. senators have serious concerns about Republican health care legislation to roll back Obamacare, moderate Republican Sen. Susan Collins, who opposes the bill, said on Sunday.

The Senate, which is delaying its consideration of the bill while Arizona Republican Sen. John McCain recuperates from surgery to remove a blood clot, will take it up as soon as all senators are available, Sen. John Cornyn, the second-ranking Republican senator, said.

McCain's absence casts doubt on whether the Senate would be able to pass legislation to repeal parts of the Affordable Care Act, Democratic former President Barack Obama's signature legislative achievement, commonly known as Obamacare.