1) It's political suicide for too many senators.
The House's bill is devastatingly unpopular, and the Senate might not do much to soften its worst impacts. Voting for such an unpopular bill, on an issue as politically powerful as health care, could be untenable for too many Republican senators. Heller and Sen. Jeff Flake (R-AZ) could face tough reelection races next year, even though the overall map is worse for Democrats, and others are already looking ahead to 2020.
Public Policy Polling came out with some horrifying numbers for Heller on Monday: 51 percent said they opposed the House bill, and 45 percent said that they'd be less likely to vote for Heller if he backed a similar bill. He trailed a generic Democratic opponent 39 percent to 46 percent.
Other Republicans face uncertain political futures. Flake is also up in 2018, in a state that's trending blue and in what could be a Democratic wave year. Collins is reportedly considering a run to be Maine's governor in 2018. Gardner faces voters again in 2020, in another increasingly Democratic state that expanded Medicaid under Obamacare.
These senators could simply decide their careers would be doomed if they were to support legislation as unpopular as what Republicans are working on.
2) McConnell won't give enough on Medicaid for the moderates.
McConnell has acceded to some of the moderate demands on Medicaid, but not all of them.If just three of the 20 Republicans who represent Medicaid expansion states refuse to support it, they could sink the bill. Portman, Capito, Murkowski, and Gardner are some of the senators most focused on the Medicaid provisions.
McConnell, according to reports weeks ago, was proposing a three-year phaseout of Medicaid expansion. Portman and the others countered with a seven-year phaseout. Yet according to Monday's report, McConnell is still set on a three-year phaseout. Can those senators really talk themselves into voting for the bill if they don't get any more concessions?
On top of that, they could also balk at the lower spending caps on Medicaid that McConnell is reportedly proposing. Portman has said repeatedly that he didn't want any deeper Medicaid cuts than the House had already included.
Capito also told me last week that one of her top priorities is protecting people on Medicaid.
"Unless I'm confident we can do that, I don't know how we can move forward in good conscience," she said.
3) The conservatives revolt because McConnell moves too far to the middle.
Outside conservative groups have been agitating over the Senate's plan, arguing it is moving too far toward the middle — a likely sign that conservatives inside the Senate are also unhappy. They're seeing McConnell delay some of the Medicaid cuts and show he's willing to keep more of Obamacare's regulations.
Paul and Lee are outwardly frustrated with the bill, and Sens. Ted Cruz (R-TX) and Tom Cotton (R-AR) could join them. They saw the archconservative House Freedom Caucus hold out for more concessions in the lower chamber, and they may decide they should do the same.
"If they don't have the votes to pass it, they're gonna have to negotiate with people," Paul said last week.
Cruz, a member of the working group that's negotiated in closed-door meetings since early May, has largely stayed positive in his comments about the Senate's talks. But in more recent days, he has started to sound a bit more pessimistic.
"We're not there yet, and we've got to get it right," he told me last week.
4) The Senate's rules make the bill untenable for too many senators.
The "budget reconciliation" process Senate Republicans are using to pass their legislation with a bare majority places some restrictions on what McConnell can include in his plan. I broke it all down last month, but the gist is that the bill is supposed to be limited to policies that directly affect federal spending or revenues.
Those rules could hamper McConnell's ability to thread this political needle. Nixing some of Obamacare's insurance regulations might not comply, which could be a deal breaker for conservatives like Cruz who are fixated on lowering premiums. They think unwinding those regulations are the best way to do it.
Or Republicans might not be allowed to defund Planned Parenthood under the reconciliation rules, another potential problem for conservatives. The House bill had also prohibited its financial assistance for private insurance from being used on plans that cover abortions. That's another policy that could be at risk under the Senate rules, which risks alienating anti-abortion senators.
Negotiations with the Senate parliamentarian, who will make these decisions, are private, so we don't know yet how real these problems are. But they are yet another complicating factor for McConnell and his pursuit of 50 votes.