Four Senate Republicans — Ted Cruz, Mike Lee, Rand Paul, and Ron Johnson — just released a statement saying they're "not ready to vote for" the Senate health care bill. That's enough to kill the bill if they actually vote against it. "It does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their healthcare costs," they wrote.
The bottom line: If a rebellion happens, it's largely because conservatives want to get rid of more of the Affordable Care Act's insurance regulations. And that's a big part of the Republican dilemma. Conservative Republicans say those rules make individual health insurance more expensive, and they're right — but the most expensive regulations are the ones that protect people with pre-existing conditions, which is exactly what Senate Republicans don't want to get rid of.