(CNN) Republicans are trying to get their plan to overhaul the US tax system through the US Senate this week, but with 52 members in the chamber, GOP leaders can afford to lose only two votes.

As the bill heads to a final vote, a small group of Republicans has yet to show their full support for the bill, and Republicans are scrambling behind the scenes to reach just the right agreement that would get to 50 votes.

Leadership made progress when they locked down Sen. John McCain as a "yes" vote, as he had previously been considered a wild card. "After careful thought and consideration, I have decided to support the Senate tax reform bill," he said in a statement.

Republicans cleared a big hurdle earlier this week when the Senate Budget Committee approved the bill -- a move that was uncertain down to the last minute since Republicans weren't sure they had the 12 committee votes they needed.

Here's where key Republicans stand:

Sen. Ron Johnson of Wisconsin

Until Tuesday afternoon, it was unclear whether Johnson would vote for the bill to clear the budget committee, where Republicans hold just a one-vote majority.

To the delight of his Republican colleagues, he ultimately voted in favor of the bill, and the committee approved it along a party line vote, 12-11. He also voted "yes" Wednesday night on a procedural vote to move the bill forward on the floor.

"We need to make some progress," Johnson told CNN when asked if he'd commit to voting for the final bill.

Johnson was the first GOP senator to say he'd vote against the plan. His concerns have surrounded the tax rate for business entities that pass through their earnings to the individual side.

Sen. Steve Daines of Montana

Daines also voted "yes" on the motion to proceed on Wednesday, but he has yet to say how he plans to vote on the final bill.

He has also shared concerns with Johnson about the pass-through rate.

Sen. Bob Corker of Tennessee

Corker, who's had serious concerns about increasing the debt, also voted for the bill in committee.

Worried, however, that it could fail to generate enough growth to pay for itself, Corker hopes to include a measure that would trigger a way to offset the costs.

Corker has been in meetings this week and said he feels optimistic about a deal, though he has not elaborated on the contours of any agreement.

"Hopefully it's going to be memorialized and part of the bill. I haven't voted for a piece of legislation yet that I thought was just outstanding," he said Wednesday. "Well, maybe. But not many."

Sen. Susan Collins of Maine

While Collins was previously considered a likely "no," she has shown a great degree of openness in the past two days on Capitol Hill. That's in part because she said she's gotten some support from the President himself for some of her proposed changes, including provisions that bolster the Obamacare marketplace.

When a reported noted Tuesday that she seemed much more supportive of the bill, Collins said "that is a fair assessment."

"A lot of my concerns it appears are going to be addressed," she added. "I'm going to be given the opportunity to offer amendments on the Senate floor."

Sen. Jeff Flake of Arizona

Like Corker, Flake is also worried about exploding the debt with the new tax reform plan. Flake has cited some of the changes that affect the middle class but disappear after a specific period as another one of his issues.

"We're still working on, you know, the deficit issues. That's my big concern," he said Tuesday. "So I think there's some progress to be made."

Sen. Marco Rubio of Florida

Rubio told reporters on Monday that he hasn't decided how he'll vote, saying there's still "not enough" progress in making child tax credits refundable for payroll workers. He's proposing an amendment with Sen. Mike Lee, R-Utah, that would make that fix, and they suggest raising the proposed corporate rate from 20% to 22% to pay for it.

The White House, however, expressed opposition to the proposal on Wednesday.

"We think that it's important to make businesses more competitive. We would not support raising the corporate rate as outlined in that amendment," principal deputy press secretary Raj Shah told reporters.

This story has been updated and will continue to update with additional developments.