On Tuesday afternoon, Senate Republicans will vote whether to start debate on a plan to overhaul Obamacare, without knowing exactly what plan they are trying to pass or, by extension, how it will affect millions of Americans.

Less than five hours before the expected first vote, the Republican game plan was still changing. Senators are now considering repealing only a few of the most unpopular provisions in Obamacare, including its individual mandate. That means there are three possibilities in play:

“Skinny repeal,” the new proposal, which would get rid of the individual mandate along with a few other unpopular provisions A full repeal of Obamacare’s spending and health insurance coverage expansion with no replacement (the Obamacare Repeal Reconciliation Act, or ORRA for short) A repeal-and-replace plan that senators have been working on for months (the Better Care Reconciliation Act, or BCRA)

The whole process has been unprecedentedly opaque as senators try to pass legislation that overhauls an industry worth more than $3 trillion and undercut a law that has extended health coverage to more than 20 million middle-class and low-income Americans in the past seven years.

The next step, Tuesday afternoon, is a vote on what’s known as the motion to proceed, which opens debate on a health care bill. It needs 50 of the 52 Senate Republicans to support it, a difficult math problem that got a little easier when Sen. John McCain unexpectedly announced he would return to Washington for the vote after a cancer diagnosis. But half a dozen Republicans are still undecided, and it remains entirely possible that the Senate will fail to even start debate.

But should that vote succeed, nobody — not even Republican senators — is sure what would happen next. The debate on the Senate floor would be open, but it is not at all clear what plan Republicans are actually trying to pass.

The big new idea: Skinny Obamacare repeal

As they struggle to wrangle together a majority, Senate leaders have started promising reluctant senators that if they pass a bill, any bill, they will go into negotiations with the House and fix the legislation there.

In order to get to conference, though, leadership needs a bill that can get 50 votes. Eliminating the penalty for Obamacare’s individual mandate — possibly along with its employer mandate and some of its taxes on the health care industry — might be the only plan that can win such broad support within the Republican conference.

Three lobbyists told Vox that this was the path forward being charted by Senate leadership. One lobbyist said the bill could be narrowed to the “lowest common denominator product.”

Such a plan faces two huge problems. First, repealing the individual mandate risks sending Obamacare’s insurance markets into a death spiral. Without a penalty for going uninsured, only people in poor health who really need health insurance end up buying it. Costs for insurers would rise, and offering plans on the exchanges would start to seem like a bad deal. The Congressional Budget Office estimated that repealing the mandate by itself would lead to 15 million fewer Americans having health insurance 10 years from now.

Second, the Republican strategy here would hinge on successful negotiations with the House to craft an entirely new health care plan. But when the House sent over its preferred Obamacare repeal-and-replace bill, a critical mass of Senate Republicans said it was unacceptable and they would never support it.

The plan to repeal and replace Obamacare is in serious trouble

Still, skinny repeal is the likely outcome if Senate Republicans can’t coalesce around a repeal-and-replace legislation. Their repeal-and-replace bill, the Better Care Reconciliation Act, was unveiled in June but has struggled to gain enough support in the Senate GOP conference.

The path for the replacement might have just gotten much harder — Sen. Rand Paul (R-KY) said on Twitter Tuesday morning that the bill would need 60 votes instead of the 50 that Republican leaders had been banking on. That would require a few Democrats to back the bill, which would never happen.

That seems to leave the bill as good as dead, if Paul is right.

Even if it were somehow kept alive, the Senate parliamentarian has deemed that several key provisions in the bill violate the chamber’s budget rules, including a provision that is essential to making the bill’s policy work.

A six-month waiting period for people who have a lapse in health coverage — the Republican replacement for Obamacare’s individual mandate — would need 60 votes to pass unless it is rewritten, the parliamentarian has said, per Senate Democrats.

There are still a lot of unanswered questions

With the vote a few hours away, here’s what we’re waiting to find out about BCRA, if leadership still has any hope of passing it:

Which version of Ted Cruz’s proposal to allow insurance companies to sell non-Obamacare plans — as long as they also sell Obamacare plans — will be included in the bill?

Is the Cruz amendment even allowable under the Senate rules? Some experts doubt it would be.

Will provisions defunding Planned Parenthood and restricting federal tax subsidies from paying for plans that cover abortions survive the process? They were also found to be in violation of the Senate rules but could be rewritten.

Will a $200 billion Medicaid “wraparound” — intended to offset the repeal of Obamacare’s Medicaid expansion (though not completely), which covered 15 million of the poorest Americans — be added to the bill?

What are the CBO’s cost and coverage projections for BCRA once the above questions are answered?

The Senate is still hurtling toward a Tuesday afternoon vote with no answers to these questions — and no idea what bill they are even trying to pass.