Sen. Marco Rubio (R-Fla.) has threatened to vote against the tax bill, putting the legislation in danger of being delayed past Christmas.

Rubio has told Senate leaders that he will vote against the bill unless the child tax credit is made more generous to help lower-income workers who pay payroll taxes and not regular income taxes.

“Sen. 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,” said a spokesperson for Rubio.

If Rubio votes against the bill, Republicans can only lose one other lawmaker if they hope to pass the final bill that emerges from a House-Senate conference.