As Republican leadership and the White House scramble to contain the fallout from a devastating Congressional Budget Office analysis that found the House G.O.P. bill to replace Obamacare would result in 24 million fewer Americans with health-insurance coverage, they are also confronting opposition from an unexpected source: Republican senator Tom Cotton of Arkansas, who on Tuesday dismissed House Speaker Paul Ryan and Health Secretary Tom Price’s plan for a multi-stage health-care overhaul as a political fantasy.

“There is no three-phase process. There is no three-step plan,” Cotton told radio host Hugh Hewitt, taking aim at earlier claims by Price that the C.B.O. report only evaluated the first of three planned phases of the G.O.P.’s Obamacare replacement. “That is just political talk,” he continued. “It’s just politicians engaging in spin.”

Cotton’s rebuke strikes at the heart of the already tenuous Republican plan, which will rely on additional pieces of legislation to bolster its initial repeal-and-replace effort. While that first phase can be passed through the budget-reconciliation process, the latter two phases—which are expected to roll back regulations and to increase competition in the health-care market, respectively—can not. And Cotton says Republicans are dreaming if they think they have the votes to pass the rest:

“Step one is a bill that can pass with 51 votes in the Senate. That’s what we're working on right now. Step two, as yet unwritten regulations by Tom Price, which is going to be subject to court challenge, and therefore, perhaps the whims of the most liberal judge in America.”

“But step three, some mythical legislation in the future that is going to garner Democratic support and help us get over 60 votes in the Senate. If we had those Democratic votes, we wouldn’t need three steps. We would just be doing that right now on this legislation altogether. That’s why it’s so important that we get this legislation right, because there is no step three. And step two is not completely under our control.”

Cotton, who has emerged as a fierce critic of the current House bill and has urged his peers to “pause” and “start over,” isn’t wrong. Much of the G.O.P.’s strategy—including what is arguably the linchpin of its plan, its “continuous coverage” provision—will rely on Democratic votes it doesn’t seem to have. Without them, the broader Republican plan is dead on arrival in the Senate—and the White House loses its ability to dismiss the current C.B.O. score by claiming it doesn’t take into account forthcoming legislation.

White House press secretary Sean Spicer attempted to shoot down Cotton’s critique during his daily briefing Tuesday afternoon, arguing that phase two is purely “administrative in nature” and phase three will ultimately draw Democratic support. “The third prong is thing Republicans have been championing for a long time, and frankly many Democrats agree with us on,” Spicer said when asked about the Arkansas senator’s objections. “I think we can not only get to 60, but I think there is bipartisan support. Because there is no one who doesn’t benefit.”

It is possible that the political calculus for Democrats will change if and when the first prong passes Congress. But as of now, Democrats remain diametrically opposed to repealing the Affordable Care Act and appear united in their pledge to resist any effort to roll back President Barack Obama’s signature health-care law. Even Senator Joe Manchin of West Virginia, who has gotten comfortable straddling the aisle and would be the most likely to join Republicans in the future, has indicated he will vote “no” on the Republican Obamacare repeal bill. “I got an older population, I got a poorer population, and I got an opiate issue we need to clean up,” Manchin told reporters on Monday night after the C.B.O. report was released. “And now, talk about insult to injury. You’ve got to have a moral compass inside of you. You can’t do that. Look at the elderly, look at the poor, look at the sick. How can you look at yourself and say, ‘O.K., I’ll help the person who needs help the least, the wealthiest people, with more tax cuts, because I’m going to be taking away from the elderly population?’ ”