The final passage of the GOP tax bill is uncertain after Republican Senator Marco Rubio said he will vote no. He will only vote yes unless the legislation includes a meaningful increase in child tax credit for working class families.

On Thursday, Sen. Rubio said, “I’ve been very consistent in my communications on this issue. I want us to see the refundable portion of the child tax credit increased from its current number. If it stays ta $1,100, I’m a no. Let’s hope it doesn’t.”

In a series of tweets, Rubio said the following:

Adding at least a few hundred $’s in refundable cuts for working families who seem to always be forgotten isn’t hard to do either 2/2

— Marco Rubio (@marcorubio) December 14, 2017

— Marco Rubio (@marcorubio) December 15, 2017

Can only support bill if % of the 2K #ChildTaxCredit available to #workingclass parents is increased to % meaningfully higher than 55% 3/3

— Marco Rubio (@marcorubio) December 15, 2017

On the other hand, Republican Sen. Mike Lee, who has been working with Rubio on the matter, remained undecided on the GOP tax bill. Lee and Rubio offered amendments to the Senate version of the bill to expand the refundable portion of the child tax credit.

Senator Bob Corker of Tennessee also rejects the GOP tax bill. He was the only Republican to vote against the original Senate version of the legislation. He remained concerned about the deficit.

On Wednesday, Sen. Corker said, "I have all the same concerns that I have had.”

Meanwhile, Sen. John McCain is currently receiving treatment for normal side effects of cancer therapy at Walter Reed Medical Center. McCain is battling an aggressive type of brain cancer called glioblastoma.

According to CNN’s sources in the Senate, the Arizona senator looked increasingly frail. During the recent GOP meetings, McCain did not speak up just the way he used to be. His energy level wasn’t the same. His potential absence in the Senate further complicates the final passage of the GOP tax bill.

The Senate Republicans can only afford to lose two votes because Vice President Mike Pence can break a tie.