Senate Republican leaders are promising to vote this week on a health care bill. That’s about all we know for sure.

But the path to advancing any legislation is an obstacle course. It could fail at multiple points — on the initial procedural vote or on final passage. An unpredictable amendment process could send the bill in unexpected directions. As of Monday morning, nobody is even sure what the end goal for Republicans is: A bill to repeal and replace Obamacare or a clean (partial) repeal bill with no replacement.

Either path faces the same problem: Senate Majority Leader Mitch McConnell (R-KY) needs 50 of the 52 Senate Republicans to back the plan under the special procedural rules Republicans are using to pass the bill with a bare majority. As of right now, the day before the expected first vote, neither bill appears to have the necessary support.

The starting point for Senate debate is the House health care bill passed in May. Eventually, Senate Republicans will swap the House bill for either the partial repeal bill or the repeal-and-replace plan — if they get that far.

The path forward is complex. Here’s what could happen next.

1) McConnell will make a motion to start debate on the House bill. That step — known as the motion to proceed — will require 51 votes. Collins and McCain mean Republican leaders are already down two votes. More moderate senators are refusing to commit to voting to start debate. Some conservatives, particularly Sen. Rand Paul (R-KY), could still balk. Vice President Mike Pence could break a 50-50 tie.

2) The Senate would debate the House legislation on the floor for 20 hours, with that time divided evenly between Republicans and Democrats. That’s 20 hours of debate time, not real time, so the debate could last a couple of days.

3) At the end of the debate, there would be a “vote-a-rama,” during which senators can offer an unlimited number of amendments to the bill. Amendments that are considered “germane” to the health care legislation need 51 votes to be added to the bill.

4) At some point, either before or after the vote-a-rama, McConnell would offer a Senate bill as a substitute for the House bill. The timing would depend on whether McConnell wants the amendments brought up during vote-a-rama to be added to the final bill or not. This will be a key decision: If McConnell waits until the end to introduce his substitute, then none of the amendments that were added during vote-a-rama will actually be part of the final legislation.

McConnell has two Senate bills at his disposal: the Obamacare Repeal Reconciliation Act, the clean (partial) repeal bill, and the Better Care Reconciliation Act, the plan to repeal and replace the health care law that Republicans have been debating for the last two months.

We still don’t know which McConnell will offer as a substitute or whether he might offer both. One speculated course of action would be for the first two amendments during vote-a-rama to be ORRA, which would likely fail, and then BCRA, which is also struggling to attract enough votes but has a more viable path to passage.

In that scenario, if BCRA did win 51 votes, the amendments that follow during vote-a-rama would change the final Senate product.

5) The Senate would take a final vote on passage of the amended Senate bill. That would require 51 votes, another dicey proposition given the estimated 22 million fewer Americans would have health insurance under BCRA. Pence could break a 50-50 tie.

6) If the Senate passes the bill, the House would probably take it up and pass it as is. The other option would be for the two chambers to negotiate a new plan, but most people in Washington expect the House to simply approve the Senate bill. A bare majority, 218 members, must vote for the bill for it to pass.

7) Once both chambers pass the Obamacare repeal bill, President Trump would sign it into law.