The GOP’s quest to repeal Obamacare is hanging by a thread, as Senate Majority Leader Mitch McConnell scrambles to lock down 50 votes just to open debate on the Senate GOP health care bill.

Two Senate Republicans – Rand Paul and Susan Collins – have said they will vote against the motion to proceed to debate. That means McConnell needs to win over every other GOP senator simply to discuss the legislation to realize Republicans’ seven-year repeal pledge.

It won’t be easy: Moderate senators like Ohio’s Rob Portman, Alaska’s Lisa Murkowski and Nevada’s Dean Heller are still on the fence about the bill, worried about its deep cuts to Medicaid. At the other end of the spectrum, conservative Sen. Mike Lee is undecided even after GOP leaders tried to include his idea for rolling back more of Obamacare’s regulations. If just one of those holdout Republicans decides to vote no, it would sink the GOP’s repeal bid and represent a massive setback for the Trump administration’s already-stalled agenda. No Senate Democrat will back it.

Medicaid expansion

If McConnell holds firm on plans to roll back Medicaid expansion over three years and put new restrictions on the program’s federal funding, it risks the support of: Jeff Flake Rob Portman Shelley Moore Capito Dean Heller Susan Collins Lisa Murkowski Jerry Moran John Hoeven Several Senate Republicans worry that overhauling the program would leave millions of vulnerable residents without coverage and force cash-strapped states to cut health services.

Obamacare regulations

If McConnell refuses to include language letting insurers sell cheap, deregulated plans alongside Obamacare-compliant coverage, it risks the support of: Ted Cruz Ron Johnson Mike Lee Rand Paul Conservatives want to give insurers the ability to sell low-cost plans with skimpy benefits alongside more generous Obamacare-compliant plans, arguing it’s the best way to lower premiums for healthy people. But it’s still unclear whether that idea would work in practice, and the insurers themselves warn it could destabilize the market.

Pre-existing conditions

If McConnell includes that provision letting insurers sell deregulated plans alongside Obamacare-compliant plans, it might risk the support of: Bill Cassidy Rob Portman Jerry Moran Lisa Murkowski Shelley Moore Capito Dean Heller Susan Collins Some moderate Republicans worry that allowing insurers to sell the deregulated plans would undermine Obamacare’s protections for people with pre-existing conditions. There’s also concern it could split the insurance markets in two – healthy people may rush to purchase the lower-cost plans with meager benefits, while sicker people are forced to purchase more generous but far more expensive coverage. Over time, that would result in prohibitive price increases for the pool with sicker people.

Planned Parenthood

If McConnell insists on retaining a provision that defunds Planned Parenthood for a year, it risks the support of : Murkowski Collins Collins and Murkowski both support maintaining the $555 million in federal funding for Planned Parenthood, and said after the Senate unveiled its initial repeal bill that they wanted to strip out its defunding provision.