If Democrats unite against the GOP’s Obamacare repeal effort, the bill can only afford to lose votes from two Republican senators. If three defect, there would be 51 votes against the bill, and it would die.

So when four Republican senators release a letter harshly criticizing how a leaked House GOP draft bill treats the Medicaid expansion, it’s a shot across the bow that could seriously endanger the chances of anything passing.

The senators — Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado, and Lisa Murkowski of Alaska — wrote that the recently leaked House repeal draft doesn’t provide “stability for individuals currently enrolled” in the Medicaid expansion.

And to emphasize how serious their concerns are, the senators wrote that while they support repealing and replacing Obamacare, they “will not support a plan that does not include stability for Medicaid expansion populations or flexibility for states.”

Now, it’s important to keep in mind that the leaked GOP draft, which was first obtained by Politico’s Paul Demko and is dated February 10, is several weeks out of date. Many changes have been made since then, and it’s possible that when the bill is released — which could be quite soon — these senators’ concerns will be met.

But as Sarah Kliff wrote recently, the leaked draft would give states “significantly less federal funding” to cover the Medicaid expansion, and would dramatically cut the Medicaid program as a whole by having the federal government pay only set amounts based on enrollment projections rather than committing to cover all medical bills that arise for enrollees.

And overall, the letter calls attention to the immense challenges Republicans will face crafting a proposal that can pass both the more ideologically conservative House and the more temperamentally cautious Senate.

Medicaid’s fate is one of the biggest things Republicans disagree on about Obamacare

In addition to its restructuring of the individual insurance markets, the Affordable Care Act expanded Medicaid eligibility to people making up to 138 percent of the federal poverty line, including childless adults, with the federal government shouldering the vast majority of the cost.

A subsequent Supreme Court ruling allowed states to reject the expansion if they wanted to. But 31 states have opted in. That means a full Obamacare repeal would eliminate health coverage for more than 10 million people who gained it through the Medicaid expansion.

Currently, 20 of the 52 Republican senators represent states that expanded the program. Several have been vocal about their lack of interest in rolling back this particular part of Obamacare. “That’s 184,000 people in my state,” West Virginia Sen. Capito, one of the signatories of this letter, told Politico last month. “That’s problematic.”

But other Republicans in Congress are from states that didn’t expand, or are ideologically opposed to continuing the expansion. “Getting ahold of the Medicaid expansion and repealing that is absolutely critical if we’re gonna have control of the growth of entitlement spending,” Rep. Bob Goodlatte (R-VA) said in leaked audio from a congressional retreat in January. Republicans in states like his, he continued, “want to be proven that they were correct originally, that we can’t afford this when we have a $20 trillion national debt.”

The House Freedom Caucus had previously demanded that any repeal bill roll back the Medicaid expansion entirely, and they may have some allies in the Senate. Meanwhile, Republican leaders had planned to use some of the savings from repealing the Medicaid expansion to fund their replacement plan — which means that if they can’t do so, they’ll have to come up with more money elsewhere.

Beyond the question of the Medicaid expansion’s fate in repeal legislation, there’s the question of what will be done with Medicaid as a whole in any replacement for Obamacare. Republicans have long proposed overhauling the program by handing over much more authority to the states to run their Medicaid programs as they see fit. The catch is that such proposals from the GOP are almost always accompanied by changes that would also cut or cap Medicaid spending — which would mean people lose coverage.

Then there’s the further hiccup that if the GOP’s Medicaid overhaul caps payments per enrollee, the states that expanded Medicaid will effectively be rewarded with more federal cash. So you get the potentially bizarre situation where, as Politico’s Jennifer Haberkorn and Rachana Pradhan report, the party might end up having to placate those non-expansion states by throwing more money at them, so it can hopefully cut the program as a whole.