WASHINGTON — Sen. Susan Collins said Sunday she “can’t envision a scenario” where she would vote for the latest Republican bill to replace Obamacare, making it likely that GOP leaders’ last-ditch effort to pass a key campaign promise will fail.

If Collins ends up opposing the bill, Republican leaders would fall at least one vote short of the 50 votes they need to pass the bill, which is sponsored by Sens. Lindsey Graham, R-S.C., and Bill Cassidy, R-La. Sens. John McCain, R-Ariz., and Rand Paul, R-Ky., have already come out against the legislation, and Sen. Lisa Murkowski, R-Alaska, is undecided.

“It’s very difficult for me to envision a scenario where I would end up voting for this bill,” Collins, R-Maine, said on CNN’s “State of the Union” talk show. “I have a number of serious reservations about it.”

Collins, who was a key vote in defeating an earlier bill in July, said she will make up her mind Monday after an analysis by the non-partisan Congressional Budget Office is released. But she said she has serious concerns that the legislation will hurt disabled children and low-income seniors who depend on Medicaid and people with pre-existing medical conditions.

“You know, my focus is on improving our health care system and what I would like to see us do is return to the very good work that the HELP (Health, Education, Labor and Pensions) Committee was doing under Senator Lamar Alexander and Senator Patty Murray,” Collins said. “I see the work we’re doing in the HELP Committee as the path forward.”

Alexander, R-Tenn., pulled the plug Tuesday on efforts to craft a bipartisan bill with Murray, D-Wash., to stabilize the individual health insurance market. Instead, he said he would support the Graham-Cassidy bill.

“I am disappointed that Republican leaders have decided to freeze this bipartisan approach and are trying to jam through a partisan Trumpcare bill, but I am confident that we can reach a deal if we keep working together — and I am committed to getting that done,” Murray said last week.

The Graham-Cassidy bill would keep most of the taxes that fund Obamacare in place but give the money to states in the form of block grants to craft their own health care insurance systems. It would also end in 2020 the expansion of the Medicaid program for low-income Americans under Obamacare.

“I don’t think block-granting Obamacare — it doesn’t make it go away,” Paul said Sunday on NBC’s Meet the Press….