Senate Republicans on Tuesday passed a motion to begin debating a replacement for the Affordable Care Act, even as it remains unclear what that replacement will actually look like.

The procedural vote, known as a “motion to proceed,” passed by a razor-thin 51 to 50 margin, requiring a tie-breaking vote by Vice President Mike Pence. All 46 Democrats and both independents in the Senate voted against it, as did Republicans Susan Collins and Lisa Murkowski.

Senator John McCain, who returned to the Senate on Tuesday after a recent diagnosis of brain cancer, cast the critical 50th vote to move forward with a mystery bill that leaves health insurance coverage for tens of millions of Americans hanging in the balance.

The vote marks the formal start of a debate process on the GOP’s plan to repeal and replace the Affordable Care Act, as they’ve promised to do for over seven years. But as of today, no one – including Republicans – knows what that plan will look like, nor whether it will have enough support to pass when it comes up for a vote.

NOW WHAT?

Over the next few days, the Senate will go through a process that includes 20 hours of open floor debate, as well as a slew of votes on procedural measures and amendments. While the process is open and somewhat unpredictable, as many as 100 different healthcare proposals could be considered by the end of the week.

“There will be a lot of different amendments offered by different members trying to craft the bill,” Senate Majority Leader Mitch McConnell said at a press conference after the vote, adding that he didn’t think it was possible to predict what specific measures would ultimately be offered or passed.

At the end of an extremely truncated debate, Republicans need to figure out what bill they want to try to pass. Like the rest of this legislative process this is likely a decision that made behind closed doors and presented to the public at the very last minute. Though Republicans often criticize the process by which the ACA was written and passed, that bill involved extensive bipartisan debate behind closed doors and in open committee over the process of multiple months. This process has shut out much of the Republican rank and file, let alone Democrats.

While we don’t know what the final bill will look like, the floor debate is expected to focus on three main proposals.

The first option is the Obamacare Repeal Reconciliation Act (ORRA), a repeal-only bill that would get rid of the Affordable Care Act in its entirety, with no plan to replace it. In a symbolic move, the GOP-controlled Congress actually passed this bill in 2015, knowing that President Obama would veto the measure (he did). But when the bill was brought up for a vote on Wednesday afternoon, Republicans weren’t so eager to pass the repeal-only measure. It failed in a 45 to 55 vote, with seven Republican senators opposing the bill.

The second option is an amended version of the Better Care Reconciliation Act (BCRA), the Senate’s latest proposal to repeal and replace the Affordable Care Act. The Congressional Budget Office (CBO) has not yet scored the amended version of the BCRA, which means the legislation would require 60 votes to pass rather than the usual 51-votes. Late Tuesday evening, the revised BRCA failed to pass the 60-vote threshold in a procedural vote.

This leaves one other option: A modified repeal bill known as “skinny repeal,” which would eliminate the Affordable Care Act’s coverage mandates and medical device tax while leaving most other provisions in place. Currently, this is considered the most likely path forward. While this may sound like a more moderate approach, the changes proposed under the “skinny repeal” bill could have catastrophic effects on the health insurance market.

THE WIDE-REACHING CONSEQUENCES OF “SKINNY REPEAL”

Getting rid of the mandate to have insurance coverage while keeping in place protections for consumers with preexisting conditions would leave very little incentive for healthy people to buy insurance. As a result, young and healthy people would start to exit the market, leaving insurance companies with a sicker and more expensive pool of enrollees, and causing a spike in costs for remaining consumers. Premiums in the individual insurance market would increase by about 20 percent relative to what they are under current law, according to the CBO. This means that by next year, average marketplace premiums would likely go up by over $1,200 if Republicans move forward with the “skinny repeal” plan.

But that’s just the beginning of a cycle that some experts say would bring the insurance market to a state of “almost complete collapse.”

With rising premiums, the cost of insurance would become unaffordable to many current enrollees. As a result, many people who have insurance through the individual marketplace would be forced to drop their coverage and become uninsured. The CBO estimates that repealing the individual mandate would result in about 15 million people losing health insurance over the next decade.

As more and more people were forced out of the health insurance market, insurance companies would eventually decide that the number of remaining enrollees was no longer worth the cost, and begin to pull out of the marketplace. But the destabilizing effects could have even more immediate consequences, given that insurance companies must make their final decisions about 2018 rates and participation in Obamacare’s exchanges in just a few weeks. Insurers are already wary about whether the Trump administration will even enforce the mandate while it is still law. As congress debates repealing the mandate altogether, these concerns are likely to grow. That could lead nervous insurers to preemptively raise premiums in anticipation of the repeal of the coverage mandates. In some cases, insurers may choose to withdraw from the individual market altogether to avoid the potential financial losses.

WHAT TO WATCH FOR

As the floor debate starts, Republicans still don’t have 50 votes on a single proposal. While “skinny repeal” is currently considered the leading contender, we really have no idea what bill Republicans will be voting on – and there’s a good chance we won’t know much until the last minute. The debate and amendment process may give us clues about what’s coming, but the real work will be done behind closed doors.

Andy Slavitt, who oversaw Medicare, Medicaid, and the Affordable Care Act under President Obama, compared the floor debate to watching a play.

Yet even among all the mystery, there’s one thing we do know for sure: Whatever comes of this process will be the product of an unprecedented maneuver to quietly railroad a major piece of legislation through the Senate without a single hearing or markup, and likely without a single Democratic amendment. As the health of millions of Americans hangs in the balance, the health of our democracy has already been downgraded.