The Republican effort to repeal Obamacare may be back from the dead, as the party makes one last attempt to get a bill through the Senate before an effective deadline of September 30.

Currently, the GOP still appears to be short of the 50 of 52 Senate votes from their own party they need to succeed (assuming all Democrats vote no). The state of play as of Wednesday midday is:

There are two likely no votes from Republican senators already: Sen. Rand Paul (R-KY) has declared his strong opposition to the new effort, and Sen. Susan Collins (R-ME) has been very critical of it as well (though she hasn’t made a final decision). If Republicans lose these two senators, they can’t afford to lose even one more from their own party.

Sen. Rand Paul (R-KY) has declared his strong opposition to the new effort, and Sen. Susan Collins (R-ME) has been very critical of it as well (though she hasn’t made a final decision). If Republicans lose these two senators, they can’t afford to lose even one more from their own party. Two other senators are noncommittal: Then, Sens. Lisa Murkowski (R-AK) and John McCain (R-AZ), are also undecided. And both of them voted against the repeal effort in July, so it’s clear they’re willing to vote down a bill they don’t like. Should Paul and Collins vote no, opposition from either one of Murkowski or McCain would doom it.

Then, Sens. Lisa Murkowski (R-AK) and John McCain (R-AZ), are also undecided. And both of them voted against the repeal effort in July, so it’s clear they’re willing to vote down a bill they don’t like. Should Paul and Collins vote no, opposition from either one of Murkowski or McCain would doom it. Several other Republican senators remain publicly undecided, but are viewed as easier to get. It’s not clear just how many votes this bill is away from passage just yet, because several other GOP senators in addition to the “big four” are saying that they’re undecided or that they’re still reviewing it. Still, Senate GOP leaders reportedly expect that these undecided senators will come around and support a bill brought to a floor vote.

The new bill in question was written by Sens. Bill Cassidy (R-LA) and Lindsey Graham (R-SC), and it would block-grant Obamacare money for states and cap Medicaid spending. It has not yet been scored by the Congressional Budget Office or been ruled as compliant with the Senate’s restrictive budget reconciliation rules. Senate Majority Leader Mitch McConnell has not yet confirmed he’ll bring the bill to a vote.

But the fast-approaching deadline of September 30 has added extra urgency to this effort. That’s the last day on which the Senate can pass Obamacare repeal with only Republican votes via the budget reconciliation process. If that date passes with no action, it would effectively kill the partisan repeal drive for the rest of this Congress. So conservatives are making one last push — since they did fall short by only one vote last time — and liberals are mobilizing to try to stop them.

So here are the key GOP senators to watch, ranked in rough order of most likely to oppose the bill to relatively less likely:

The big four

1) Rand Paul (R-KY): In recent days, Paul has become the Senate’s most vocal critic of Cassidy-Graham, tweeting repeatedly that he’d oppose the bill because it keeps too much of Obamacare in place. Still, there’s been some skepticism about whether he’d truly be willing to be the swing vote who kept Obamacare in place. (Last time, he voted against McConnell’s initial repeal-and-replace bill but then did come around to vote yes on the Senate’s “skinny repeal” bill, which was essentially a placeholder to keep the repeal effort alive.)

On Monday afternoon, Paul held a press conference in which he reiterated his opposition to the bill. According to Vox’s Tara Golshan, when Paul was asked if any changes to Cassidy-Graham could get his vote, he said he didn’t “see any possibility” that that could happen.

2) Susan Collins (R-ME): Collins opposed all three versions of repeal that were brought to a vote in July. She’s a longtime senator with a moderate reputation who represents a purple state. Given her past votes and her political situation, most expect she’ll vote against this repeal effort too.

On Monday night, Collins released a negative statement on Graham-Cassidy, saying she had several “concerns” with it. On Tuesday morning, she said she was “leaning no” on the bill.

3) Lisa Murkowski (R-AK): Murkowski also opposed all three versions of repeal that came before the Senate in July. Her situation is a bit more complex than Collins’s, though. For one, Alaska is a conservative state. But Murkowski has an independent streak, even surviving a primary defeat in 2010 to win reelection via write-in campaign. Additionally, Alaska is an unusual state facing very specific health market challenges. Theoretically, that could mean some special allowance could be made in the bill to help out Alaska, but that didn’t work last time.

Murkowski has said all week that she’s undecided on Cassidy-Graham. On Wednesday afternoon, she said she needed to see more data about how the bill would impact her state.

4) John McCain (R-AZ): McCain shockingly tanked the Obamacare repeal effort in July by voting against skinny repeal, calling for his colleagues to return to regular order and pursue a bipartisan effort. But Cassidy-Graham is co-written by his closest friend in the Senate, Lindsey Graham, and McCain’s public comments on the matter have been less than definitive. He said Monday he might “reluctantly” back the bill if his state’s governor, Doug Ducey, endorsed it — and hours later, Ducey did.

On Monday afternoon, McCain told reporters that he was “not supportive of the bill yet,” and reiterated his previous complaints about the rushed process, lack of hearings, lack of amendments, and lack of regular order. He didn’t quite say he was a definite no, but he was clearly still quite unhappy with the process.

On Wednesday afternoon, McCain reiterated that his position hasn’t yet changed and that he wants regular order. “Nothing has changed. If McConnell wants to put it on the floor, that’s up to McConnell," he told Politico’s Burgess Everett.

Other GOP senators may also have concerns

Beyond these four holdouts, there are other GOP senators who have expressed concerns about various Obamacare repeal proposals in the past, or who voted against either McConnell’s repeal-and-replace bill or the repeal-and-delay bill in July.

These senators have demonstrated a pattern of backing Republican leaders on tough votes in the end — they all put aside their concerns to back skinny repeal — so they’re considered less likely to vote no on Cassidy-Graham too.

Still, it’s worth remembering that some of these votes were won over because the skinny repeal bill was sold as a placeholder merely meant to keep the process alive. This time around, these senators would have to be voting for a version of Obamacare repeal that would be likely to become law with zero further changes — so there’s no more wiggle room. These senators are:

Shelley Moore Capito (R-WV): Capito represents a state that benefits greatly from Obamacare’s Medicaid expansion. But her state is also extremely conservative and very pro-Trump, meaning she could be vulnerable to backlash from the right for bucking the president. In July, Capito ended up voting yes on McConnell’s initial health bill (though that version of the bill was clearly doomed), no on the repeal-and-delay bill that didn’t provide any replacement for Obamacare, and yes on the skinny repeal bill.

Capito represents a state that benefits greatly from Obamacare’s Medicaid expansion. But her state is also extremely conservative and very pro-Trump, meaning she could be vulnerable to backlash from the right for bucking the president. In July, Capito ended up voting yes on McConnell’s initial health bill (though that version of the bill was clearly doomed), no on the repeal-and-delay bill that didn’t provide any replacement for Obamacare, and yes on the skinny repeal bill. Rob Portman (R-OH): Portman is in a similar situation to Capito — his state benefits from the Medicaid expansion, and he voted yes on McConnell’s repeal-and-replace bill and the skinny bill but no on repeal-and-delay — but overall he’s close to Republican leadership, and it’s not clear he’d defy them here.

Portman is in a similar situation to Capito — his state benefits from the Medicaid expansion, and he voted yes on McConnell’s repeal-and-replace bill and the skinny bill but no on repeal-and-delay — but overall he’s close to Republican leadership, and it’s not clear he’d defy them here. Lamar Alexander (R-TN) : Alexander, who chairs the Senate’s health committee, has preferred bipartisan efforts to stabilize Obamacare rather than a partisan drive to repeal it. But he kept his public criticisms of the partisan effort to a minimum, and he did end up voting yes on McConnell’s bill and the skinny bill.

Alexander, who chairs the Senate’s health committee, has preferred bipartisan efforts to stabilize Obamacare rather than a partisan drive to repeal it. But he kept his public criticisms of the partisan effort to a minimum, and he did end up voting yes on McConnell’s bill and the skinny bill. Mike Lee (R-UT) : Like Rand Paul, Lee is a frequent critic of Obamacare repeal proposals from the right, arguing that they don’t go far enough. But Lee, too, will surely be looking at that September 30 deadline and weighing a vote for this bill versus leaving all of Obamacare in place.

Like Rand Paul, Lee is a frequent critic of Obamacare repeal proposals from the right, arguing that they don’t go far enough. But Lee, too, will surely be looking at that September 30 deadline and weighing a vote for this bill versus leaving all of Obamacare in place. Jerry Moran (R-KS): Another senator from a deep red state, Moran has been critical of some of the GOP’s repeal proposals. It was his defection alongside Lee’s that made clear that Mitch McConnell’s repeal-and-replace plan would fail. However, he did support skinny repeal in the end. Moran is currently undecided on Cassidy-Graham.

If any of these senators comes out against Cassidy-Graham, it would be devastating for the bill’s chances. But Collins, Murkowski, Paul, and McCain seem more likely to be the key swing votes.