Sens. Marco Rubio, R-Fla., and Mike Lee, R-Utah, are withholding their support for the GOP tax bill in an attempt to bolster the child tax credit, a change that may be difficult amidst opposition from House Republicans.

The duo is hoping to make the credit fully refundable. The two senators, backed by top White House adviser Ivanka Trump, had previously succeeded in increasing the credit to $2,000 per child in the Senate-passed bill.

Both members previously voted for the bill. Sen. Bob Corker of Tennessee was the lone Republican to vote against it over concerns with the measure’s impact on the deficit, and he has been working with the administration to try to address those concerns.

Should Lee and Rubio join Corker in opposing it, the bill would fail. The GOP can only afford to lose the support of two members in the event of a tie-breaking vote from Vice President Mike Pence.

“I understand this is a process of give and take, especially when there are only a couple of us fighting for it. Your leverage is lessened,” Rubio said. “But given all of the other changes they made in the tax code leading in to it, I can’t in good conscience support it unless we are able to increase the refundable portion of it. And there are ways to do it,” he continued.

A spokesman for Lee said the Utah Republican is now “undecided” on the legislation.

“He continues to work to make the [child tax credit] as beneficial as possible to American working families,” the spokesman said.

A Republican lawmaker, speaking on background to discuss internal negotiations, said it was unlikely the changes sought by Rubio and Lee could be made given opposition in the House.

“The House pushed back hard on that,” the member said of the revised child tax credit in the Senate bill. “We’re pretty much done with that.”

Jennifer Shutt contributed to this story.