Senate Republicans will move forward with a vote to begin debate on a health care bill Tuesday afternoon, but many observers — and even the senators themselves — are confused about which version of the bill Senate Majority Leader Mitch McConnell will actually bring to the floor.

You can watch live on Facebook shortly after 2 pm EDT.

There is an added layer of drama as reports emerged late Monday night that Sen. John McCain, who has been in the hospital in Arizona with a blood clot and recent diagnosis of a form of brain cancer, will return to Washington today. Republicans had been short of the 50 votes needed to proceed without him.

What is going to the floor?

The Senate is currently working with two versions of a health care bill: the Better Care Reconciliation Act — a version of the House bill that would repeal and replace the Affordable Care Act — and the Obamacare Repeal Reconciliation Act, which would repeal first and delay replacement by two years. The Congressional Budget Office has projected if the BCRA passes, 22 million people would lose coverage, and if the ORRA is passed, 32 million people would lose coverage.

As Vox’s Dylan Scott explains, there are actually two critical votes that could happen: the “motion to proceed,” which opens up Senate floor debate, and, after that, a vote on one or both of the Senate bills.

How will the vote work?

McConnell needs at least 50 votes to pass his health care agenda. The GOP has a narrow majority in the Senate (52 seats) and is facing a united Democratic Party, laser-focused on ending the Republican effort to undermine Obamacare. With McCain’s surprising return to Washington, McConnell has a slightly better chance of pushing something through. McCain has said publicly that he would vote to advance the bill, though he has been critical of the process in passing the bill.

Senate leaders were planning to rely on a reconciliation strategy, which only requires a simple majority of 51 votes instead of the usual 60 needed for legislation with a budgetary element to be approved. Any amendments to the bill must also survive the Byrd Rule once debate begins, and many of the ideas to bring senators on board may not.

The Senate parliamentarian declared on Friday that certain aspects of the BCRA, including Planned Parenthood defunding and restrictions on abortion coverage, would still require that 60-vote approval. Leadership is looking at other options for preserving these provisions in the final bill.

If the motion to proceed goes through, the Senate would debate the legislation on the floor for 20 hours, with that time divided evenly between Republicans and Democrats. As Scott points out, “that’s 20 hours of debate time, not real time, so the debate could last a couple of days.”

After that debate period, there would be what’s called a “vote-a-rama,” where senators can offer unlimited amendments on the bill, which then needs 51 votes to clear. Some Republican senators have said they intend to vote the bill into debate despite not knowing exactly what it contains.

Who are the major players?

Sen. Susan Collins (R-ME), a centrist, is most likely a “no” on this bill. She’s objected to past iterations of the proposal on the grounds that it doesn’t cover enough people and includes harsh Medicaid cuts that would harm her most vulnerable constituents. She appeared on Face the Nation Sunday saying of this vote, “I don't think that's a good approach to facing legislation that affects millions of people and one-sixth of our economy.”

Sen. Rand Paul (R-KY) is also reportedly a “no,” which, until McCain’s return, seemed to be enough to stop the bill from advancing. Paul’s reasoning, however, is that the bill doesn’t go far enough to dismantle the Affordable Care Act.

With McCain back in the chamber, dissent from both Sens. Collins and Paul wouldn’t be enough to kill the effort; however, there are other voices that could add nails to the coffin. Sen. Mike Lee (R-UT) takes issue with an amendment by Sen. Ted Cruz (R-TX) that allows non-Obamacare plans to leak back into the market, and Sen. Jerry Moran (R-KS) has previously expressed his distaste for the BCRA. Sens. Lisa Murkowski (R-AK), Shelley Moore Capito (R-WV), and Rob Portman (R-OH) have also spoken out against the Medicaid cuts.

How to watch:

Time: The vote will be taken Tuesday reportedly around 2:30 pm on the Senate floor, following a closed-door meeting at 12:45 pm.

Live stream: Vox video on Facebook