After months of negotiations, House and Senate Republicans have produced separate versions of a sweeping tax package with dozens of provisions that would cut some $1.7 trillion in taxes on businesses and individuals.
With a Senate vote tentatively set for Thursday, a few GOP holdouts are looking to make last-minute deals.
Senate Majority Leader Mitch McConnell of Kentucky has been struggling to whittle down the list of holdouts in his caucus to win approval for the measure. With details of the final bill still in flux Thursday, it was unclear if he had the required 50 votes nailed down.
Here are the latest positions of nine Senators who have expressed concerns about the bill in the run-up to Thursday's floor debate of the bill.
Sen. John McCain of Arizona, whose dramatic "thumbs down" vote helped doom the GOP effort to repeal Obamacare, said earlier this week that he wanted to wait for the final version of the tax-cut bill before announcing his position.
But as the Senate debate got underway Thursday, with various amendments likely, McCain's office said he was now in the "yes" camp.
"After careful thought and consideration, I have decided to support the Senate tax reform bill. I believe this legislation, though far from perfect, would enhance American competitiveness, boost the economy, and provide long overdue tax relief for middle class families," McCain said in a statement.
Sen. Lisa Murkowski of Alaska, another GOP opponent of her party's failed Obamacare repeal effort in July, has said she will not oppose the provision in the Senate tax bill repealing the individual mandate.
Murkowski's support for the tax bill is due in large part to a separate bill that would open up federal lands in Alaska, the Arctic National Wildlife Refuge, to oil and gas drilling. The two bills are being merged as part of the process of writing a final bill for approval by both the House and Senate.
"We still have work to do on this legislation," Murkowski said in a statement Thursday, "and I look forward to the debate on the Senate floor and my colleagues' ideas to further improve it."
Sen. James Lankford of Oklahoma is a conservative Republican who has opposed the tax bill, based on its impact on the national debt. But earlier this week, his office also said that Lankford was eager to work with his caucus to improve the bill.
On Thursday, he had apparently come around to a "yes" vote.
"Sen. Lankford supports the bill and anticipates several changes to address his debt concerns," his spokesman told CNBC.
Sen. Steve Daines, a Montana Republican, has expressed concerns about the so-called "pass-through" treatment of business income.
On Monday, Daines appeared to be moving toward a "yes" vote after Tweeting that he had spent the weekend speaking with the White House about his concerns.
But as of Thursday morning, he was still undecided, his office said.
Sen. Ron Johnson of Wisconsin has taken a firm stand against the Senate bill, which has to be merged with the House version before it can be brought to the floor for a vote.
Johnson, a former chief executive of a small manufacturing company, has said the legislation unfairly favors large corporations over small businesses.
"In the current form, I wouldn't vote for it," he told CNBC last week.
But Johnson has also said he would work with GOP leadership and the White House to make changes in the bill that would allow him to support it.
As the debate got underway Thursday, his office did not immediately reply to a CNBC request for comment.
Sen. Susan Collins, a moderate from Maine, was among three Republicans who blocked her party's effort to repeal Obamacare in July. As of Thursday, she was still undecided on the tax bill.
She is still working to address two main concerns: the tax bill's repeal of the mandate requiring individuals to buy health insurance, a move which could undercut Obamacare, and the elimination of individual deductions for state and local taxes.
It would be "very difficult for me to support the bill if I do not prevail on those two issues," Collins told reporters Thursday morning before the debate began
But she said she was encouraged by her discussions with Senate colleagues about the changes she wants. She's asked lawmakers to include provisions in a separate bill that would continue Obamacare subsidy payments for low income people and help insurers cover expensive patients.
Collins also wants the final bill to include a provision, already approved by the House, which would retain a local property tax deduction of up to a $10,000 limit. She is also pressing for a refundable tax credit for adult dependent care, paid for by closing a loophole on carried interest.
Collins also said she thinks the corporate tax rate doesn't need to be cut as low as 20 percent, as President Donald Trump has favored. She said 21 or 22 percent would be "fine with me."
Sen. Bob Corker of Tennessee, a Trump critic who has decided not to run for re-election, is a longtime deficit hawk who was among those raising major concerns about the tax bill's impact on the national debt. As originally written, the Senate bill would add $1.5 trillion to the national debt, not counting interest.
"We're $20 trillion in debt and its party like there's no tomorrow time in Washington," Corker tweeted when the bill won approval from the Finance Committee earlier this month.
The bill's GOP proponents maintain that the tax cuts will spur economic growth that will generate enough new revenues to offset the initial cuts, an argument few independent analysts are buying. That includes the Congressional Joint Committee on Taxation, which projects a $1.5 trillion increase in deficits over ten years, not counting the interest on the debt needed to pay for it.
To win over Corker and other deficit hawks, Republicans added a "backstop" to curb future budget deficits if economic growth promised by the bill's authors doesn't materialize. The details have yet to be spelled out, but the so-called "trigger" would restore some tax cuts if the plan doesn't boost the U.S. economy enough to make up for lost revenues.
Despite the lack of details, the change seems to have won Corker over, who voted Tuesday to advance the bill from the Budget Committee to the full Senate.
"While we are still working to finalize the details I am encouraged by our discussions,"Wednesday.
While the inclusion of the "trigger" could allay Corker's concerns and win his vote, it could cost the bill support from several other GOP senators who have spoken out against the idea, including Nevada Sen. Dean Heller and Louisiana Sen. John Neely Kennedy.
Sen. Jeff Flake of Arizona, another vocal GOP critic of Trump who is not seeking re-election in 2018, has also expressed concern about the tax bill's impact on the national debt.
He has also reportedly voiced concerns that the bill could end up costing more than currently projected because tax cuts that are set to expire in several years could be extended and cost the government even more in lost revenues.
As of Thursday morning, he was reportedly undecided on the bill pending details of the "trigger" mechanism.
Sen. Jerry Moran of Kansas has also expressed concerns about the impact of tax cuts on the national debt.
"We don't want to increase the debt and deficit as a result of tax cuts," he told a town hall meeting in Kansas on Saturday. "My goal is to find out which taxes you cut can actually help create more jobs, better jobs, higher-paying jobs … and which ones don't do that. Not all of them do that."
Kansas is still reeling from a round of tax cuts in 2012 that were later reversed after the measure blew a large hole in the state's budget.
On Tuesday, a spokesman said Moran "is determined to pass tax reform and the Senator continues to work with his colleagues to accomplish that."