Good question — it's going to be a tough vote.
There are two groups threatening to vote it down for different reasons: Conservatives and moderates. As of Wednesday night, at least 27 Republicans were either hard "no" or leaning "no" by an NBC News count. That would kill the bill as GOP leaders can afford only 22 defectors, since no Democrats are expected to support the bill.
Conservatives, led by the House Freedom Caucus, say the bill preserves too many elements of Obamacare for them to support it and many of the top advocacy groups on the right, like Heritage Action and Club For Growth, are egging them on. Many would prefer to repeal Obamacare entirely (or at least more of it) as a starting point rather than jump into a replacement immediately.
Moderates have the opposite concern. They worry the bill is too radical and that it cause millions to lose health insurance. Some are worried that its cuts to Medicaid will hurt their state budgets. Addressing their problems would mean further angering conservatives and vice versa.
That doesn't mean the bill is dead, though. It's difficult to vote against your own party's president and Trump is aggressively pushing members to get on board.
On Wednesday night, House leaders and the White House were talking with the House Freedom Caucus about a possible deal to eliminate a major Obamacare provision called "Essential Health Benefits." These are 10 broad categories of care, like hospitalization and maternity care, that the law requires insurers cover.
Conservatives want to drop these requirements, which would allow insurers to offer plans that cover fewer items but have more affordable premiums as a result. Critics are worried insurers will use these cheap plans to attract customers who are already healthy while customers with chronic health issues won't be able to find an affordable plan that covers their needs. Politically, this issue could be more sensitive due to the current focus on the opioid addiction: EHB's currently require plans to cover treatment for substance abuse.
It gets more complicated, though. House Republicans are using a procedure called budget reconciliation to pass the bill because it requires only a bare majority in the Senate, rather than 60 votes. The downside is that it limits what Congress is allowed to include in the bill. Initially House GOP leaders argued changes to Essential Health Benefits would not pass muster with the Senate parliamentarian, but they seem more willing to test the waters now.