(CNN) - Sen. Marco Rubio, a Republican from Florida, told congressional leadership that he will not support the latest version of the GOP tax bill if the legislation doesn't make the child tax credit more generous.
"I've been pretty consistent in my communications on this issue and that is I want us to see the refundable portion of the child tax credit increased from its current number," Rubio told reporters on Capitol Hill on Thursday. "If it stays at $1,100, I'm a 'no.' Let's hope it doesn't."
First reported by The Washington Post on Thursday, Rubio is a potential problem as the Senate GOP prepares to bring a tax bill to the floor for a vote next week.
Republican leaders are working with an already thin majority in the Senate. Majority Leader Mitch McConnell can only afford to lose two votes and already Sen. Bob Corker, a Republican from Tennessee, voted against the last version of the bill. Corker said Wednesday he still maintained many of the same concerns from his last vote with the most recent version of the bill.
"Senator Rubio has consistently communicated to the Senate tax negotiators that his vote on final passage would depend on whether the refundability of the Child Tax Credit was increased in a meaningful way," Olivia Perez-Cubas, an aide to Rubio, told CNN in a statement Thursday.
Despite the very real revenue issues Republicans face on their tax bill, Senate GOP leaders are working to put more money toward the refundability piece of the child tax credit, according to two GOP sources involved in the process. The amount is currently being negotiated and it's unclear Rubio will get exactly what he wants, but he will get something, the sources said.
Rubio -- along with Sen. Mike Lee of Utah -- has been working for months to make the child tax credit more refundable, something the senators argue will help lower-income Americans, but so far their efforts haven't convinced leaders who are trying to craft a tax bill that still meets Senate rules and doesn't cost more than $1.5 trillion. What Lee and Rubio are asking for is expensive, but Rubio has publicly expressed frustration that GOP leaders were willing to increase the corporate tax rate to 21% to pay for lowering the top tax rate for individuals, but not for helping lower-income people with children.
Lee is undecided right now on the tax bill, according to an aide.
"Sen. Lee is undecided on the bill in its current forms," spokesman Conn Carroll told CNN in a statement. "Sen. Lee continues to work to make the CTC as beneficial as possible to American working families."
The White House said Thursday they would continue working with Rubio on the child tax credit, but insisted they'd already made substantial progress on the topic as negotiators finalize a tax deal.
"We think he should be very exited about the progress made on that front," White House press secretary Sarah Sanders said.
She said the White House was "very proud" of the work they'd already accomplished with the Florida Republican, calling it "historic movement."
Sanders couldn't say whether Trump had spoken recently with Rubio about tax reform, and said the White House expected tax reform to pass before the end of the year.
Another complicating factor is that two Republican senators John McCain of Arizona and Thad Cochran of Mississippi are dealing with ongoing health issues.
This story has been updated and will continue to update with additional developments.
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