Senate Majority Leader Mitch McConnell (R-Ky.) stands by Sens. Lindsey O. Graham (R-S.C.) and Bill Cassidy (R-La.) during a news conference on Capitol Hill about the Cassidy-Graham bill on Sept. 19. (Melina Mara/The Washington Post)

Senate Republicans’ latest effort to kill Obamacare has been nothing if not abrupt.

Think about it: At this time last week, Sens. Lindsey O. Graham (R-S.C.) and Bill Cassidy (R-La.) hadn’t formally announced their bill devolving federal health-care spending to the states. Even when they did, congressional news led with President Trump’s talks with Democratic leaders on protecting undocumented immigrants. A sudden GOP gambit on health care seemed like a remote possibility, at best.

That all changed over the weekend when Republican leaders, pushed by conservatives to more seriously reckon with the possibility that they would not achieve their party’s central policy goal, began to take a closer look at the Cassidy-Graham bill. And now, they face the challenge of persuading 50 people in the Senate to support it before the end of the month, which would set the stage for Vice President Pence to cast the tiebreaking vote.

There are many questions surrounding this process. But the timing is perhaps the chief source of confusion among congressional observers. Why is it necessary to pass the health-care bill by Oct. 1? Why do Republicans say they have to act in the next 11 days?

The answer lies in a combination of Republican legislative strategy, arcane Senate procedure and ordinary partisan divisions. But it boils down to the fact that on Oct. 1, the number of votes needed to pass Cassidy-Graham will rise above the number of Republicans in the Senate, effectively blocking the bill’s path.

Senate Republicans are trying to revive the momentum to overhaul the Affordable Care Act with the Cassidy-Graham proposal. Here's a rundown of the plan, and the rush to pass it. (Jenny Starrs/The Washington Post)

Here’s one way to think about it.

The Senate has until Sept. 30 to approve Cassidy-Graham with a simple majority of votes rather than the usual 60 needed for most legislation. This creates a valuable opportunity for the GOP and its slim, 52-seat Senate majority. From now until the end of the month, Senate Republicans can approve legislation fulfilling one of their most controversial priorities without a single Democratic vote.

Of course, uniting 50 Republicans is hardly simple thanks to divisions within the party, and it’s still unclear whether GOP leaders can do it for Cassidy-Graham in time to meet the deadline. That, coupled with seven years of Republican commitments to repeal the health-care law, is why pressure on the leaders is so high.

“If we were going to go forward, we would have to act before Sept. 30,” Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday at a news conference. “We’re in the process of discussing all of this. Everybody knows that the opportunity expires at the end of the month.”

McConnell and other Republicans can thank themselves for the deadline, which arose from their effort to pass health-care legislation without Democratic votes.

This is where the arcane Senate procedure comes in.

The Sept. 30 deadline exists because of a process known as budget reconciliation, which allows some fiscal measures to pass without the usual 60 votes. Republicans set this process in motion at the beginning of the year, when they passed a budget bill that included instructions for two committees to begin work on health-care legislation with the goal of saving federal revenue. By giving the health-care effort a fiscal goal, GOP leaders qualified that legislation to be passed by a simple majority.

But those instructions expire at the end of the fiscal year that’s covered under the budget bill. Senators could always write new instructions into their next budget, but they were planning to use that opportunity to direct a different legislative priority — tax cuts. Conventional Senate wisdom dictates that the chamber may consider only one legislative priority at a time under reconciliation.

Republicans would prefer to face no deadline at all. But these hopes were dashed on Sept. 1, when the Senate parliamentarian, who helps interpret the chamber’s mind-bending rules, said the GOP’s “reconciliation instructions” would end Sept. 30, the last day of the fiscal year. That is what McConnell mean when he said the opportunity will “expire” at the end of the month: Senate Republicans will no longer have the ability to pass health-care legislation with only their own votes, because the process that allows them to do it will be over.

It’s for this reason that so many journalists and commentators refer to Cassidy-Graham as Republicans’ “last-ditch effort” to abolish the Affordable Care Act, also known as Obamacare.

Graham urged Republicans to try again if his bill doesn’t pass.

“At the end of the day, will we try again? I would argue that yes, we should. I don’t want to go back to South Carolina and say I did everything I could do to repeal Obamacare and not believe it in my heart,” he said Tuesday.