Multiple Republican senators appeared reluctant to support the GOP's current Obamacare replacement bill Monday following the release of a Congressional Budget Office report assessing its effects, putting its passage this week in doubt.

The plan would lead to 22 million more Americans uninsured in 2026, while average premiums are expected to fall after climbing at first, according to the nonpartisan CBO's estimate. It would lead to an estimated $321 billion in deficit reduction from 2017 to 2026.

Senate Majority Leader Mitch McConnell, who praised the CBO's findings, is pushing for a vote on the bill this week. A procedural vote to proceed with the plan could come as soon as Wednesday. Republicans, who hold 52 Senate seats, can only lose two votes and still pass the bill.

Multiple skeptical GOP senators appeared unimpressed by the proposal on Monday night, raising more doubts about the plan's ability to pass this week without tweaks. Six Republicans have now publicly opposed the proposal as written, though some of those votes could change if the plan is revised.

Moderate Republican Susan Collins of Maine said she would vote "no" on the motion to proceed with the bill. In a tweet, Collins said she wants to work with Republicans and Democrats to "fix the flaws" in Obamacare, but the "Senate bill won't do it."

I want to work w/ my GOP & Dem colleagues to fix the flaws in ACA. CBO analysis shows Senate bill won't do it. I will vote no on mtp. 1/3

Conservative GOP Sen. Rand Paul of Kentucky, one of senators to previously announce opposition to the plan as written, said Monday that he will not vote for the bill to proceed unless it changes, according to NBC News. He previously argued the proposal did not go far enough to repeal Obamacare.

GOP Sen. Ron Johnson of Wisconsin, another conservative who announced he would not back the current bill, said Monday he "would highly doubt" he would back a motion to proceed, according to NBC.

Republicans face difficulties in winning over skeptical senators, as tweaks to appease conservatives could alienate moderates, or vice versa.

After the CBO report's release, Sen. Bill Cassidy, R-La., told CNN that he had not seen the CBO report but that, as described by the network, it made him "more concerned." He added that he remains "uncommitted."

Sen. Dean Heller, R- Nev., already announced that he would not support the bill as written, criticizing in strong terms its rollback of Medicaid expansion. Heller, a moderate, is up for re-election next year in Nevada, a state where Medicaid expansion provided coverage to about 210,000 people, according to its Republican Gov. Brian Sandoval.

The GOP could still win skeptical senators over with last-minute amendments. House Republicans did the same to gather more votes before the chamber narrowly passed its own Obamacare replacement last month.