WASHINGTON (Reuters) - A proposal by U.S. Republicans to repeal and replace the Obamacare health insurance program suffered new setbacks on Sunday, when two pivotal senators from the party dug in with criticisms of the bill that President Donald Trump is pushing for quick passage.

Senator Susan Collins, who just two days ago said she was "leaning against" the legislation, on Sunday declared in an interview on CNN's "State of the Union": "It is very difficult for me to envision a scenario where I would end up voting for this bill."

Collins said her concerns centered on the impact the legislation would have on the federal Medicaid program that helps disabled children and low-income elderly people get healthcare.

Trump and Senate Majority Leader Mitch McConnell can afford to lose the support of only two Republicans, assuming all Democrats vote against the measure to repeal and replace the Affordable Care Act better known as Obamacare.

RELATED: Republicans who voted 'No' on repeal of Obamacare

7 PHOTOS Republicans who voted 'No' on repeal of Obamacare See Gallery Republicans who voted 'No' on repeal of Obamacare Sen. Susan Collins, R-Maine (Photo by Mark Wilson/Getty Images) Sen. Dean Heller, R-Nev. (Photo By Bill Clark/CQ Roll Call) Sen. Lisa Murkowski, R-AK (Photo by Chip Somodevilla/Getty Images) Sen. Lamar Alexander, R-Texas (Photo by Zach Gibson/Bloomberg via Getty Images) Sen. Shelley Moore Capito, R-WV (Photo by Alex Wong/Getty Images) Senator John McCain, R-Ariz. (Photo via REUTERS/Aaron P. Bernstein) Sen. Rob Portman, R-Ohio (Photo via REUTERS/John Sommers II) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

Senators John McCain and Rand Paul already have expressed opposition.

Paul, interviewed on NBC's "Meet the Press," attacked the centerpiece of the Republican bill that would have the federal government basically turn the health insurance system over to states in the form of "block grants."

“They could remove the block grants from it, and we can vote on what we actually on agree on,” Paul said. “I can’t in good conscience vote to keep all the spending."

Despite the difficult odds in fulfilling their seven-year drive to kill Obamacare, some key Senate Republicans were pushing to forge ahead.

Senator Lindsey Graham, who is leading the charge on the latest version of Obamacare repeal, told ABC: "The only way you know how people will vote is you have the vote," adding, "I think we're going to get the votes."

The Senate faces a Sept. 30 deadline for deciding on the bill under an expiring rule that lets the healthcare proposal pass with just a simple majority, instead of the 60-vote threshold needed for most legislation.

Republicans hold a 52-48 majority in the Senate. If two Republican senators vote against the bill, Vice President Mike Pence would probably cast the deciding, tie-breaker vote to win passage.

A third Republican "no" vote sank a different version of Obamacare repeal in July.

(Reporting by Susan Cornwell and Richard Cowan; Additional reporting by Sarah Lynch and Valerie Volcovici; Editing by Lisa Von Ahn)

More from :

The 'fatal flaw' in the new Obamacare-repeal bill

Trump: I assumed when I won, I would 'sit down at my desk and there would be a healthcare bill'

Obama derides GOP for trying to undo 'hard-won progress' on health care