The absence of two GOP senators is creating uncertainty about the timing of the chamber’s vote on tax legislation.

Sens. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) and Thad Cochran William (Thad) Thad CochranEspy wins Mississippi Senate Democratic primary Bottom Line Mike Espy announces Mississippi Senate bid MORE (R-Miss.) have both missed Senate votes this week, and one of their votes will likely be needed to pass the bill.

Republican leadership signaled on Thursday that the exact timing of the vote, and which chamber will move first, is now being hashed out.

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"I don't know the answer to that question. It's all about timing and managing absences in the Senate. ... We're simply being flexible to honor their concerns about managing their schedule and some possible absences," Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) told reporters.

The Senate was expected to move first on the tax deal, with an initial vote expected as soon as Monday. The upper chamber would then pass the measure and send it to the House on Tuesday for a final vote.

But Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas), the No. 2 Senate Republican, said on Thursday that there are ongoing discussions about which chamber will move first.

"I think it's just a matter of logistics," Cornyn said, asked what was putting the timing into limbo.

Senate GOP leadership is confident that they will have the votes to pass the tax deal next week, before leaving Washington for the year.

But they face a narrow path with a 52-seat majority, meaning they can only afford to lose two GOP senators and still have Vice President Pence break a tie.

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Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) voted against the Senate's tax bill and indicated this week that he's undecided on the conference report, saying it doesn't address his concerns about the deficit.

If Corker remains a "no" on the tax bill, GOP leadership would need either McCain or Cochran’s vote for passage.

Complicating matters further, Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (R-Fla.) is warning that he could vote against the bill unless the child tax credit is expanded.

"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," a spokeswoman for Rubio said on Thursday.

Cornyn reiterated on Thursday that he expects McCain to return next week for the tax vote.

"He's just saving his energy and resting up for the big vote next week," he told The Hill on Wednesday evening, asked if McCain's absence would impact the tax vote.

McCain was diagnosed with brain cancer over the summer. His office said this week that he was at Walter Reed Medical Center receiving treatment "for normal side effects of his ongoing cancer therapy."

Meanwhile, Cochran's office said on Thursday that he would vote on both the tax bill and a short-term continuing resolution to fund the government next week.

Cochran had an outpatient procedure Monday related to a nonmelanoma lesion on his nose.

Both senators are expected to be "yes" votes on the tax bill.

Pence's office announced on Thursday that he would delay his trip to the Middle East to preside over the Senate vote.

"Yesterday the White House informed Senate leadership that due to the historic nature of the vote in the Senate on tax cuts for millions of Americans, the vice president would stay to preside over the vote," Pence press secretary Alyssa Farah said in a statement.

Pence also presided over the Senate's initial tax vote, even though it was clear his vote would not be needed.

- This story was updated at 2:10 p.m.