The change appears to be a victory for Mr. Rubio, who said on Thursday he would not vote for the bill unless it included a more generous credit that would allow families without any federal tax liability to claim the credit for each child.
Mr. Rubio wanted to raise the amount that is refundable above 55 percent by what he calls a meaningful amount. Whether the 70 percent now contained in the bill will be enough to gain his support remains to be seen.
The bill had not been released publicly as of Friday morning. But the text was scheduled to be viewable by Democratic and Republican negotiators from both chambers as of 10 a.m., in a room closed off to reporters and even the lawmakers’ own staff.
On Friday, President Trump told reporters he had seen the bill, and he liked it.
“I have seen it,” Mr. Trump said in brief remarks at the White House. “I think it’s going to do very, very well. I think that we are going to be in a position to pass something as early as next week, which will be monumental.”
Even if he were to oppose the final bill, Mr. Rubio would not by himself stop the legislation from making its way to Mr. Trump’s desk, as Republicans hope to do next week; the bill passed the Senate with one Republican vote to spare, and if Mr. Rubio were to defect, Vice President Mike Pence could still break a tie in the bill’s favor.
If other Republicans were to also break ranks, the bill would lack the votes needed for passage. At least two other Republican senators remained publicly undecided on Friday morning: Mike Lee of Utah, who has allied with Mr. Rubio in pressing for an expanded child credit, and Susan Collins of Maine, who has expressed reservations about the bill’s reduction in the top individual tax rate and pushed for party leaders to support measures to bolster individual health care markets as a condition for her vote.
Ms. Collins office said on Friday that she had not yet seen the final bill.
Mr. Trump was asked about Mr. Lee and Mr. Rubio on Friday, and he expressed no concern about their support.
“I think they’ll be great,” he said. “They’re great people. They want to see it done. I know them very well. I know how they feel. These are great people and they want to see it done, and they want to see it done properly.”