Sen. John McCain announced Friday he will not vote for a last-ditch effort to overhaul Obamacare, imperiling passage before a critical Sept. 30 deadline.

The Arizona Republican said the bill, authored by Sens. Lindsey Graham, R-S.C., and Bill Cassidy, R-La., did not receive sufficient consideration through hearings and markups in the Senate.

"I cannot in good conscience vote for the Graham-Cassidy proposal," McCain said in a statement. "I believe we could do better working together, Republicans and Democrats, and have not yet really tried."

The GOP effort on healthcare would transfer revenue from Obamacare to states in the form of block grants so they can set up their own healthcare systems. It was being advanced by Republicans as they faced the end of September deadline to use reconciliation, which requires 50 votes for passage rather than the 60 needed to block a filibuster.

The bill's sponsors, Republican leadership, and the White House have been pressing hard to win the 50 votes needed. But with Sen. Rand Paul of Kentucky repeatedly saying he would vote against it and Sen. Susan Collins of Maine saying Friday she was leaning against it, McCain's no vote would kill the bill.

In July, McCain flew into Washington after learning that he had brain cancer and cast an unexpected, deciding vote against a bill that would have narrowly repealed parts of Obamacare, which was known as "skinny repeal." Collins, who repeatedly has voted against efforts to repeal Obamacare, and Sen. Lisa Murkowski of Alaska, also voted against the skinny repeal. Murkowski has not said how she would vote on the current legislation, leading to heavy pressure from the White House and Republican leaders.

"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," McCain said in his Friday statement. "But that has not been the case. Instead, the specter of Sept. 30 budget reconciliation deadline has hung over this entire process."

The Senate Finance Committee has scheduled a hearing for the Graham-Cassidy bill for Monday.

Rather than work on the Graham-Cassidy bill, McCain encouraged his fellow senators to work on a bipartisan agreement that had begun in the Senate Health, Education, Labor and Pensions Committee. The work involved four hearings held this month aimed at finding ways to stabilize the Obamacare exchanges, which are facing the prospect of higher premiums for unsubsidized customers in 2018, as well as reduced options for consumers after insurers left the markets in the face of uncertainty and financial losses.

Democrats have pushed for additional funding for Obamacare, while Republicans have asked for states to have more flexibility both in how they implement healthcare programs and how quickly their proposals can be approved by the federal government.

Sen. Lamar Alexander, HELP committee chairman, said this week that the efforts were abandoned because he could not reach a consensus with Democrats. Sen. Patty Murray, the committee's top Democrat, said she still remained hopeful they could arrive at an agreement.

After McCain's announcement, she said she agreed the process should take place through "regular order and working together to find common ground."

"I'm still at the table ready to keep working, and I remain confident that we can reach a bipartisan agreement as soon as this latest partisan approach by Republican leaders is finally set aside," she said. Republicans raised doubts about Democrats' sincerity to work on a bipartisan effort after Sen. Bernie Sanders, I-Vt., introduced a bill with 16 Democrats that would move all Americans into Medicare, even if they have private insurance. Democrats, meanwhile, said they grew distrustful of Republicans after the introduction of Graham-Cassidy and after more support grew to overhaul Obamacare.

McCain discouraged such divisions in his statement.

"Senators Alexander and Murray have been negotiating in good faith to fix some of the problems with Obamacare," McCain said. "But I fear that the prospect of one last attempt at a strictly Republican bill has left the impression that their efforts cannot succeed. I hope they will resume their work should this last attempt at a partisan solution fail."