Senate Republicans on Sunday unveiled the latest version of an Obamacare overhaul bill as they face a five-day window for passage and dwindling support within their own party.

Three GOP senators came out against the former version of the bill, including Rand Paul of Kentucky, John McCain of Arizona and Ted Cruz of Texas. Sen. Susan Collins, R-Maine, has not explicitly said she would vote against the bill, instead conveying she is leaning against it and it would be difficult for her to envision a scenario in which it would earn her support.

The legislation, which has commonly come to be known as Graham-Cassidy, was authored by GOP Sens. Lindsey Graham of South Carolina, Bill Cassidy of Louisiana, Dean Heller of Nevada and Ron Johnson of Wisconsin. It would transfer Obamacare's revenue to states in the form of block grants and would repeal the individual and employer mandates.

Critics of the bill say it does not provide states with enough time or federal funding to craft their own healthcare plans, and have pointed out that though the bill calls for health insurance coverage to be "affordable," it does not specify what percentage of income that means. Critics also worry coverage will become prohibitively expensive for people with pre-existing illnesses like cancer or diabetes.

The latest version of the bill projects increased in federal funding to votes from swing states Arizona, Kentucky and Alaska, which would have seen reductions under the previous version. It also requires states to describe how they would address pre-existing illnesses.

On Sunday night, Senate Minority Leader Chuck Schumer slammed the latest version of the legislation.

"Despite an attempt to appear to add money for a select few states, this bill is just as bad for those states and the rest of the states because it still contains a massive cut to Medicaid, and would throw our health insurance system into chaos while raising premiums," he said in a statement. "It still takes away protections for those with preexisting conditions and further weakens consumer protections." Disagreements within the Republican Party have been similar to past versions of bills that would have repealed parts of Obamacare. Conservatives like Paul and Cruz believed the initial version of the bill didn't go far enough in repealing Obamacare and reducing the cost of health insurance, and it's unclear whether the latest version will sway them. Centrists like Collins believe the solution to the healthcare system should be established through bipartisan debate.

And the timeline to win over McCain, who cast the deciding vote against the "skinny repeal" bill in August, has run short. McCain has called for "regular order," which he defines as multiple hearings followed by open debate and amendments from both sides. The sole hearing scheduled on Graham-Cassidy is for Monday afternoon in the Senate Finance Committee and involves testimonies from federal and state health officials.

Republicans have aimed to advance the healthcare bill through reconciliation, which requires a simple majority that can include support from Vice President Mike Pence. Because they have only a two-seat majority in the upper chamber, and no Democrat will vote for the legislation, then losing the support of three senators would doom the bill. They also face a deadline of Sept. 30 this fiscal year as recommended by the Senate parliamentarian.

Senate Majority Leader Mitch McConnell has said he intends to bring a bill to the floor for a vote this week, but it has not yet been scheduled. He and other Republicans have pointed to Democratic backing of a bill recently introduced by Sen. Bernie Sanders, I-Vt., along with 16 Democrats, to move all Americans from private coverage to a government-funded and run healthcare system, as evidence they should revisit efforts on Obamacare.

"I've come to the conclusion that Obamacare is a placeholder for Berniecare and the Democratic world," Graham said Sunday. "The best you could hope for is to prop up Obamacare because they are moving toward Berniecare."

Further complicating GOP efforts is Sept. 30 is also the deadline for re-authorization of the Children's Health Insurance Program, or CHIP, which must occur every five years. The joint federal-state program, which has been in place for two decades, provides health insurance coverage 9 million low- and middle-income children.

The deadline for insurers to sign contracts in most states is also this week, on Sept. 27, and many insurers have either pulled out of the Obamacare exchanges for next year, reducing options for customers, while others have requested steep rate increases.

The Graham-Cassidy bill has not yet been scored by the Congressional Budget Office, the agency that will determine how much the legislation would cost the federal government and whether more people will become uninsured. A full score will not be available this week in time for the vote, and Cassidy has said the first score also will be outdated after a more recent version of the legislation is introduced.

If Republicans fail to pass a bill this week, Graham suggested he would like them to have another shot at repealing parts of Obamacare in 2018, which is a midterm election year. Graham, who is on the Budget Committee, said he would press for passing another resolution on the budget that includes healthcare. This would allow for a second pass at reconciliation, which Republicans had planned to use to pass a tax reform bill.

Trump appeared to echo similar sentiments in comments about healthcare to reporters on Sunday, saying: "Eventually we'll win, whether it's now or later."