Sen. Susan Collins, a Republican from Maine, speaks to members of the media in the basement of the U.S. Capitol in Washington, D.C.

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

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

Collins is one of two Republican senators who have already said that they would not even vote to open debate on the latest version of the bill released on Thursday, meaning one more defection from the Republican ranks could kill it.

Republicans control the Senate by a 52-48 margin. With the Democrats solidly opposed to the legislation, the Republicans can only pass the bill if all their other members back it and if Republican Vice President Mike Pence casts his tie-breaking vote in favor.

A Washington Post-ABC News poll published on Sunday showed Americans preferred Obamacare by a 2-1 margin. Approaching six months in office, Trump's overall approval rating has dropped to 36 percent from 42 percent in April.

While Collins said that she did not know if the legislation would ultimately pass, she said as many as 10 Republicans have doubts about it.