Sen. John McCain, R-Ariz., on Friday said he "cannot in good conscience vote" for the latest GOP Obamacare repeal plan.

McCain is one of four Republican senators who have been undecided on the proposal, known as the Graham-Cassidy bill, and his opposition dealt the bill's chances a significant blow. Health care stocks jumped on the news.

In a statement, McCain said his opposition stemmed in large part from how rapidly the bill, nicknamed for Republican Sens. Lindsey Graham of South Carolina and Bill Cassidy of Louisiana, was being pushed through the Senate in order to meet a September 30 procedural deadline.

McCain said he cannot vote for the bill without knowing how it will affect premiums, how much it will cost, and how many people it would help or hurt.

"Without a full CBO score, which won't be available by the end of the month, we won't have reliable answers to any of those questions." he said.

McCain also noted that, in theory, the bill could be a good piece of legislation. "I would consider supporting legislation similar to that offered by my friends Senators Graham and Cassidy were it the product of extensive hearings, debate and amendment. But that has not been the case," he said.

Also on Friday, Sen. Susan Collins, R-Maine, said she is leaning toward voting no on Graham-Cassidy. Kentucky Sen. Rand Paul has already expressed his opposition to the bill, which he said didn't fully repeal the Affordable Care Act.

The bill would not pass if Collins, McCain and Paul voted against it. Republicans have until Sept. 30 to pass a health care bill with a simple majority. After that, they will need 60 votes, a nearly impossible threshold to meet as Democrats have been united in opposition to the Republican Obamacare repeal efforts.

The fourth undecided vote is Lisa Murkowski, R-Alaska, who has expressed her concerns about the bill to fellow senators. Senate Majority Leader Mitch McConnell has pledged to bring Graham-Cassidy to the floor for a vote before the end of the month.

McCain's decision to oppose Graham-Cassidy also has a personal component: Graham is the Arizona lawmaker's best friend in the Senate, and many in Washington mistakenly believed that this decades-long friendship would help to sway McCain's vote.

Shortly after McCain released his statement Friday, Graham publicly reassured his friend that there were no hard feelings.

Graham tweet.

The White House was not so forgiving. "A vote against Graham Cassidy is a vote to save Obamacare," said Vice President Mike Pence, speaking in his home state of Indiana.

"This is not going to be easy," he said of the faltering repeal effort. "But let me be clear. The Republican majority in Congress was not elected to save Obamacare, they were elected to repeal and replace it. So it's time for every member of the Republican majority to keep their word to the American people."

The remarks seemed to point squarely at McCain, but the Trump administration has good reason to worry. McCain's opposition significantly reduced the likelihood that Graham-Cassidy will garner enough votes to pass. And this slimmer chance of success could prompt other senators who've supported the bill so far to flip their yes votes to nays.

Later Friday, White House director of legislative affairs Marc Short responded to McCain's statement, saying that Republicans will continue to push for success in repealing Obamacare.

"At this point, we're only two votes away. We'll continue working to get the votes of Susan Collins and Lisa Murkowksi," Short said.

When asked if there could potentially be a deal sweetener for Alaska, Short said "there isn't such a thing in this bill." He also said it was too soon to talk about a bipartisan effort.

Two senators to watch on this front are Ted Cruz, R-Texas, and Mike Lee, R-Utah, both of whom have previously expressed concerns that Graham-Cassidy doesn't go far enough toward fully repealing Obamacare.

Now that there's blood in the water, conservatives like Cruz and Lee could decide that voting "no" on Graham-Cassidy will burnish their credentials with conservative voters more than voting yes on an imperfect bill that ultimately collapses.

If Republicans fail to repeal or replace the Affordable Care Act, they will effectively break a promise that GOP lawmakers have been making to voters ever since Obamacare was first signed into law in 2010. President Donald Trump campaigned on repealing and replacing the law, as well.