The current major item of legislation, the Republican tax overhaul bill, suddenly appears to be imperiled in a truly unique number of ways. It’s by no means dead, mind you, but far more stumbling blocks are presenting themselves now than in the recent past.

This is the composition of the Senate at large, as of Thursday. There are 46 Democrats, who have shown no indication that they plan to aid the Republicans’ tax efforts. There are two independents who caucus with the Democrats and are similarly unwavering. And there are 52 Republicans — plus Vice President Pence, who gets to cast a vote in the event of a tie in the chamber.

When the Senate’s initial draft of the tax bill passed earlier this month, the vote occurred on party lines with the exception of Sen. Bob Corker (R-Tenn.). The retiring thorn in President Trump’s side voted no on the measure, making the final tally 51-49 (and leaving Pence on the sidelines).

On Thursday, though, another defection: Sen. Marco Rubio (R-Fla.) announced his opposition to the bill as written. He wants an expansion of the child tax credit, which his peers have rejected. If Rubio stays a no, the bill can still pass, thanks to Pence. But the Republicans could not lose anyone else.

The problem is that Sen. Mike Lee (R-Utah) has already started to wobble. He, too, supported expanding the child tax credit and is officially undecided on the final bill. With him hovering in the middle, the Republicans have only 49 solid votes, meaning that the plan is at risk of failing.

It gets worse. Two Republican senators are experiencing health issues at the moment. Sen. John McCain (R-Ariz.) is at Walter Reed Medical Center dealing with the effects of the treatment he has undergoing for brain cancer. Sen. Thad Cochran (R-Miss.) has been battling health issues all year, including minor surgery earlier this week. Both Cochran and McCain say that they will be available for votes next week, but, as Cochran himself can attest, those are hard commitments to make with absolute certainty.

Meanwhile, there’s another deadline looming that seemed unlikely just a few days ago. At some point — perhaps as late as Jan. 3 but possibly as early as Dec. 27 — the results of Tuesday’s special Senate election in Alabama will be certified. At that point, Doug Jones will be sworn in to replace Sen. Luther Strange (R-Ala.), who was appointed to fill the term of Jeff Sessions after he was tapped to be Trump’s attorney general. If you’ve just emerged from a six-month hibernation, Jones is a Democrat. If Rubio balks, Cochran and McCain can’t vote, Lee abstains and Jones is in the chamber, the vote is a lopsided 51 to 46 against the measure.

That’s a lot of ifs, and most of them will probably be resolved. Rubio may be appeased by a concession in the bill, as may Lee. McCain and Cochran may, as expected, be able to cast a vote — while Strange is still a senator.