The fate of the last-ditch push to repeal and replace Obamacare will come down to one impossible question: How fickle will Republican senators be on their health care positions?

On Monday, Sen. Rand Paul (R-KY) voiced clear and staunch opposition to the Cassidy-Graham-Heller-Johnson proposal — the last remaining Obamacare repeal plan that would block-grant Obamacare funding, cap federal health care spending, and send the money to the states to come up with their own health care programs.

“It keeps 90 percent of Obamacare and redistributes the proceeds,” Paul said in a meeting with reporters in his office Monday. He continued:

I don’t think anybody has realized the enormity of this. Obamacare took a long time to get in place. It took them a year to get their website. Can you imagine now every state has got to go through this? Start completely over with all the subsidies. Some states might want subsidies, some won’t, some states might go to single-payer. I think it will be a chaotic nature for two years. It’s not repeal. It’s another incarnation of Republican replace. But not repeal.

As the GOP faces a September 30 deadline on a Republican-led Obamacare repeal effort, Paul’s opposition to the bill could stand in the way once and for all.

Republicans can afford to lose two votes on the Cassidy-Graham-Heller-Johnson health proposal. The vote will come down to four key Republican senators, including Paul, as Vox’s Andrew Prokop wrote. The other three are Sens. John McCain (AZ), Lisa Murkowski (AK), and Susan Collins (ME).

Republicans who want to support the plan will have to go back on what they said in the past

It’s hard to envision a way for Paul to dig himself out of a “no” after his comments Monday. He said he doesn’t “see any possibility” in which he could sign up for this bill, adding that he doesn’t see how his colleagues who previously expressed concerns with past iterations of the Obamacare repeal plans could sign on to the proposal either.

Supporting the plan would certainly require Republicans to backtrack on objections they’ve raised throughout the eight-month quest to repeal Obamacare.

Collins said in the past that she would not support a health care bill that has a per capita cap on Medicaid funding — which Graham-Cassidy does.

McCain said he could “reluctantly” support it — before later telling reporters he wants to see any health bill go through regular order and months of debate, which Cassidy-Graham will not in the Senate.

“There were many people in the first iteration that were concerned about Medicaid ... that’s all still in there,” Paul said. “So we will see if the people that were concerned about the slowdown of growth in Medicaid — how they respond. Several of them are in states that will get less money because of the formula. And the formula was just taken arbitrarily out of space — mostly to take money from more Democratic states and give it to Republican states.”

Among other Republican senators who may come out with dissenting opinions are Shelley Moore Capito (WV) and Rob Portman (OH), who come from states that expanded Medicaid under the Affordable Care Act and would see drastic cuts in federal health care spending under this proposal. (Arizona would also see federal funding cuts, but Gov. Doug Ducey endorsed the proposal Monday.)

Soon after Paul’s announcement, other senators’ concerns became apparent. Sen. John Kennedy (R-LA) told reporters he’s worried that block-granting money to states could allow Democratic states to push further toward single-payer — a concern that is difficult to reconcile with how much the block granting structure would cut from states like New York and California.

Another Obamacare repeal push was always going to be an uphill battle, especially after the dramatic floor votes that saw three repeal proposals voted down.

Even so, this week has brought a sense of momentum behind the proposal — one that Paul is concerned could even lead to the proposal’s passage, even as he said he didn’t see how his colleagues could support it.

“I’m worried,” Paul said. “There is a big groundswell of people pushing for this.”