Sen. Marco Rubio is withholding support for the Republican tax bill unless it is changed to extend the enlarged child tax credit to low-income families.

A “no” vote from Rubio could endanger the bill, which passed the Senate with only 51 votes, including his. Republicans have only a two-vote majority in the Senate.

The Florida Republican has called for making the $2,000 child tax credit included in the bill refundable against payroll taxes, a change that would benefit many low-income families who don't have income tax liability.

“If it stays at $1,100, I'm a ‘no,’” Rubio told reporters, referring to the portion of the credit that would be refundable under the Senate-passed version of the bill. “But let's hope it doesn't.”

Rubio also suggested that he would accept a compromise that increased the refundable portion of the credit but not all the way to $2,000, as he has proposed.

“I understand this is a process of give and take,” he said, adding that so far he has received no assurances from leadership.

On Wednesday night, Sen. Mike Lee, R-Utah, said that the final bill was likely to make the credit partially refundable, but that negotiations are continuing.

Despite Rubio’s announcement, GOP whip John Cornyn said Republicans are still on track to pass the bill by Tuesday.

“We are working with him. I think he’ll be satisfied.”

The difficulty facing Republicans is that they limited themselves through budget rules to a tax cut of $1.5 trillion over 10 years. That means that the expanded credit would have to be offset elsewhere in the bill.

One possibility would be to compromise on the corporate tax rate. On Senate passage, Rubio and Lee had offered an amendment to make the credit fully refundable against payroll taxes by setting the corporate tax rate at 20.94 percent rather than 20 percent. That amendment was rebuffed.

In conference, however, Republicans have settled on a corporate tax rate of 21 percent to pay for other priorities, including a lower top individual tax rate.

"If you found the money to lower the top rate...you can't find a little bit to at least somewhat increase the refundable portion of it?" Rubio asked rhetorically.

Senate Finance Committee Chairman Orrin Hatch called Rubio’s stance “a problem.”

“It will be interesting to see what can be done” to accommodate Rubio, the Utah Republican said.

President Trump also downplayed Rubio's opposition during brief comments at an event at the White House, saying that the tax bill would pass before Christmas. Rubio has "really been a great guy, very supportive," he said. "I think that Sen. Rubio will be there."

Republicans were aiming to finalize the tax overhaul bill Friday to pass it next week.

• Al Weaver contributed to this report.