Republicans on Thursday were putting the final touches on their tax code overhaul ahead of likely votes next week, but potential pitfalls remain, including health-related absences or new opposition by key senators.

House and Senate GOP tax writers expect to unveil a conference agreement Friday, followed by votes as soon as Monday in the Senate and potentially Tuesday in the House. Senate Majority Whip John Cornyn, R-Texas, said Wednesday that Republicans were still on track to take it up Monday.

House Ways and Means Chairman Kevin Brady, R-Texas, told reporters he expects to make the conference report available for signatures between 10 a.m. and 12 p.m. Friday, with public release “later in the day.”

But the timeline for votes could hinge on whether two ailing Republican senators, Thad Cochran of Mississippi and John McCain of Arizona, will be back to work by Monday. Both senators have missed votes this week due to health issues: Cochran had an outpatient procedure to remove a non-melanoma lesion on his nose and McCain has been receiving treatment at Walter Reed Medical Center for “normal side effects of his ongoing cancer therapy.”

Speaker Paul D. Ryan, R-Wis., said Thursday that Senate absences were indeed a factor as Republican leaders tried to nail down a timeline for finishing the tax overhaul before lawmakers leave for the holidays.

Ryan, Pelosi Talk Government Funding End-Game

“It’s all about timing and managing absences in the Senate,” Ryan said at a news conference. “I’ve talked to [Senate Majority Leader Mitch McConnell] a couple of times about this. We’re simply being flexible to honor their concerns about managing their schedule and some possible absences.”

Even if all senators are present, the vote will still be tight. Vice President Mike Penceannounced Thursday he will delay a trip to the Middle East in order to preside over the Senate next week, where he might be needed to break a tie vote on the tax bill.

Sen. Bob Corker of Tennessee was the only Republican to vote against the Senate version of the tax bill when it passed 51-49 earlier this month. Corker had concerns with the price tag of the tax plan, which is projected to add more than $1 trillion to annual deficits over the next decade.

Other GOP defections are still possible. Sen. Marco Rubio, R-Fla., has pushed for the final House-Senate agreement to expand refundability of the child tax credit and has said there could be “problems” if those changes aren’t included.

Rubio told reporters he needs to see an increase in the House and Senate bills’ current limit of $1,100 in 2018 on refundable child credits for households without enough tax liability for the nonrefundable portion to offset.

“It certainly has to be higher than it is now in both houses,” Rubio said. “Right now it’s only $1,100. It needs to be higher than that,” he added, while noting the refundable portion slowly grows with inflation but he wants to see a larger base amount.

Additionally, Sen. Mike Lee, R-Utah is “undecided” at this point because of the child tax credit issue, a spokesman said.

House and Senate Republicans reached a tentative agreement Wednesday on broad details of the final tax plan, including a proposed 21 percent corporate tax rate and a top individual tax rate of 37 percent.

Further details continued to leak out Thursday, including proposed repatriation rates higher than those included in the House and Senate bills. That change would help offset revenue losses from other changes to the tax plan.

Sen. Ron Johnson, R-Wis., said he thinks the conference report will set the rates at 8 percent for non-liquid funds and 15 percent for cash assets, up from 14.5 percent and 7.5 percent respectively in the Senate version and 14 percent and 7 percent in the House-passed bill.

Joe Williams, Jennifer Shutt and Lindsey McPherson contributed to this report.