With the clock ticking, Senate plans to vote on new Obamacare repeal bill next week

The Senate plans to vote on a bill that would repeal and replace Obamacare next week, amid uncertain support from lawmakers and opposition from medical groups and health insurers.

"It is the Leader's intention to consider Graham-Cassidy on the floor next week," said McConnell Spokesman David Popp Wednesday afternoon, referring to the bill by the names of its co-sponsors.

Support for the Graham-Cassidy bill, considered to be Republican's last shot to follow through with a campaign promise to undo Obamacare, hinges on a handful of Senators who have not yet indicated how they plan to vote.

Senate Republicans face a September 30 deadline to pass their bill with a simple majority of 51 votes, meaning the bill could be killed if more than two Senators vote 'no.'

Sen. Rand Paul, R-Ky, told ABC News he's "leaning" towards no, turning the spotlight on Senators John McCain, R-AZ, Lisa Murkowski, R-AK, and Susan Collins, R-Maine, who are the most likely to affect the outcome of next week's vote.

Despite the uncertainty surrounding next week's vote, President Donald Trump told reporters today he thinks Graham-Cassidy has a "very good chance."

"I believe that Graham Cassidy will do it the right way," said Trump, adding that it has "tremendous support from Republicans."

Graham-Cassidy works by redistributing federal funding from the Affordable Care Act to states in the form of block grants, halting Medicaid expansion, and repealing the individual and employer mandates in Obamacare. States could also apply for waivers to alter what counts as "essential health benefits," like maternity care. They could also obtain waivers to charge people with pre-existing conditions more.

Supportive Republicans say this bill gives states the power to cover their constituents according to their needs.

"I would not sign Graham-Cassidy if it did not include coverage of pre-existing conditions. It does! Great Bill. Repeal and Replace," Trump tweeted Wednesday night.

But Democrats they're concerned about people losing coverage.

They've also voiced concerns -- shared by McCain -- about the lack of a full Congressional Budget Office scoring of the bill that would signal how many people could lose coverage and how much the bill could cost, and the lack of hearings.

"We had a chance to do something important, historic, bipartisan that really would have built on the health care system which we have in this country and now we're moving towards another partisan approach, no CBO score, no hearings, getting ready to repeat what we did just a few months ago," Sen. Dick Durbin, D-IL, told ABC News' Mary Bruce.

But Democrats aren't alone in their concerns. Today, health insurance companies that have remained largely quiet about Graham-Cassidy came out in opposition. In a letter to Senate leadership, American Health Insurance Plans, a powerful trade organization for health insurance companies including Anthem, Cigna and Humana, say the bill does not stand up to their list of health care principles including stabilizing the individual insurance markets, and guaranteeing access to coverage for consumers, including those with pre-existing conditions. Medical organizations offered similar concerns about potential loss of coverage, including the American Hospitals Association, and American Medical Association.

With the clock ticking, the Senate Finance committee will hold a full hearing on the bill next Monday.

ABC News' Mary Bruce and Ali Rogin contributed to this report.