Herb Jackson and Ledyard King

USA TODAY

WASHINGTON — With a Republican plan to overhaul taxes hanging by a one-vote thread, Sen. Marco Rubio, R-Fl., said Thursday he would oppose the bill next week unless the child tax credit is expanded further to help working-class families who don't owe income taxes.

Other Republican senators also were undecided, including Mike Lee of Utah, who co-sponsored the child tax credit proposal with Rubio. Vice President Mike Pence changed his plans for a Mideast trip next week so he could be in Washington if needed to cast a tie-breaking vote.

Republican leaders in the House and Senate said Wednesday they had an agreement in principle on how to merge competing bills that both chambers passed in recent weeks. A House-Senate conference committee is expected to release a bill Friday afternoon, and votes expected on Monday and Tuesday.

But when the Senate approved the bill on Dec. 2, the vote was 51-49, meaning Majority Leader Mitch McConnell, R-Ky., could only afford to lose one vote and still pass it with Pence breaking a 50-50 tie.

Lee and Rubio voted for the earlier bill, even after the Senate rejected an amendment they had offered to increase the child credit and allow recipients to get the full amount even if they do not owe income taxes. They proposed to offset the cost by increasing the top corporate tax rate from 20% to just under 21%.

The agreement reached this week between Republican leaders of the two houses and President Trump's administration did include a 21% corporate rate, but the funds were partially used to lower the top individual tax rate to 37%, from 38.5% — without expanding the child credit from the earlier Senate bill.

Rubio reacted bitterly to that news, questioning on Twitter why it was considered "anti-growth" to raise the rate to pay for a tax cut for "a working family making $40k" but not for couples making $1 million.

Despite the reported agreement, details of the bill are still being written and it's possible Lee and Rubio could be accommodated. Trump's daughter and aide, Ivanka Trump, has also pushed for the expanded credit and met with Senate leaders on Wednesday.

Conn Carroll, a spokesman for Lee, said the senator "is undecided on the bill in its current form. (He) continues to work to make the (tax credit) as beneficial as possible to American working families."

Speaking to reporters on Thursday, President Trump did not seem concerned about losing Rubio’s vote.

“I think he’ll get there. He’s really been a great guy, very supportive,” Trump said. “I think that Sen. Rubio will be there, very shortly.”

Currently, the credit is $1,000 per child, and it begins to shrink as a couple's income exceeds $120,000. The Senate bill would increase it to $2,000, and increase the income phase-out to $500,000. But only the first $1,100 would be refundable, meaning a taxpayer could receive a check for that amount even if they did not owe income tax.

Rubio has said the full amount should be refundable.

"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," spokeswoman Olivia Perez-Cubas said in a statement released Thursday afternoon.

Fellow GOP senators pushed back against Rubio's threat, saying they prevailed in negotiations with the House by boosting the credit to $2,000 (the House wanted $1,600) and keeping the refundability portion at $1,100.

“We fought hard in the conference between the House and the Senate to hold our position, because, as you know, the House didn’t like what we put in the Senate in terms of the refundability increase or the increase in the child tax credit generally,” said Sen. Rob Portman, an Ohio Republican and one of the negotiators. “We’ll we’ve already won. We should take our victory.”

Sen. John Thune, R-S.D., a member of the GOP leadership, said the fight is over.

"It has been negotiated," he told reporters. "I think we’re probably where we’re going to end up.”

Rubio and Lee are not the only Republican senators wavering. Sens. Susan Collins of Maine and Jeff Flake of Arizona are undecided. Both voted for the bill after getting assurances on unrelated issues: For Collins, it was action on health insurance measures, while for Flake, it was a role in immigration policy.

Adding to the uncertainty about the tax vote is the health of Sen. John McCain, R-Ariz. McCain’s office said Wednesday he was admitted to Walter Reed Medical Center for treatment of “normal side effects of his ongoing cancer therapy” and would return to work “as soon as possible.”

Sen. Thad Cochran, R-Miss., also has missed votes this week due to medical reasons.

Contributing: Deirdre Shesgreen and Eliza Collins.