Senate Majority Leader Mitch McConnell (R-KY) (C) talks to reporters with (L-R) Sen. Cory Gardner (R-CO), Sen. John Barrosso (R-WY) and Sen. John Thune (R-SD) following their party's weekly policy luncheon at the U.S. Capitol May 16, 2017 in Washington, DC.

Senate Majority Leader Mitch McConnell wants to pass the GOP's Obamacare replacement this week, but the plan is in serious danger of stalling before it even comes to a vote.

As of Tuesday morning, four Republican senators — enough to block a procedural motion to proceed with the bill — have said they will oppose the motion barring changes to the plan. A Congressional Budget Office score on Monday saying that the proposal would lead to 22 million more uninsured Americans by 2026 only complicated matters for moderate GOP senators on the fence.

GOP Sen. Susan Collins of Maine, a swing vote, said Monday night she will vote "no" on the motion to proceed, tweeting that the Senate bill does not "fix the flaws" of Obamacare. She joined Sen. Dean Heller, a vulnerable Nevada Republican who previously said he would vote against advancing the bill as written due to its rollback of Medicaid expansion.

I want to work w/ my GOP & Dem colleagues to fix the flaws in ACA. CBO analysis shows Senate bill won't do it. I will vote no on mtp. 1/3

On the conservative side, Sens. Rand Paul of Kentucky and Ron Johnson of Wisconsin also said they would not back the motion to proceed this week for the bill as written. They argue that the plan does not go far enough to repeal Obamacare.

"I won't vote to proceed to it unless the bill changes," Paul told reporters on Monday, noting that he wants to negotiate.