Senate Republican leaders want to pass their version of the Tax Cuts and Jobs Act (TCJA) by the end of the week.

There are a slew of Republicans, however, who still have serious concerns about the bill.

GOP leaders can only afford to lose two votes for the TCJA to pass.

The Republican tax plan is in the midst of a critical week in the Senate, but it’s unclear if GOP leaders have the votes to pass their ambitious package.

Senate Majority Leader Mitch McConnell and Republican leaders say they want to bring the Tax Cuts and Jobs Act (TCJA) to the Senate floor for a vote by the end of the week. That would keep the TCJA on track to meet President Donald Trump’s self-imposed Christmas deadline for final passage.

While leadership may have an ambitious timeline, a sizeable group of GOP senators have expressed concerns about the bill since its release just more than two weeks ago.

Their issues range from the tax legislation’s treatment of small businesses, to healthcare, to the federal deficit. Since the bill can only lose two Republican votes for it to pass, the margin for error is incredibly thin.

Below we’ve collected the eight senators to watch in the next week and why they could spoil Republican leaders’ best-laid plans.

Ron Johnson of Wisconsin

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Foto: source AP

Why he could vote no: Johnson was the first public “no” on the TCJA. He has raised concerns about the bill’s treatment of smaller pass-through businesses, such as S-corporations and limited liability companies, when compared to large corporations. According to reports, GOP leaders are attempting to tweak the refund to pass-throughs to win over Johnson.

Susan Collins of Maine

Foto: Sen. Susan Collins of Maine. source Alex Wong/Getty Images

Why she could vote no: Collins has bucked GOP leadership repeatedly, most notably over the plan to repeal and replace the Affordable Care Act. Collins is not a fan of repealing Obamacare’s individual mandate in the bill – nor how it would make individual tax cuts temporary. Collins is seen as one of the hardest members to win over.

Jeff Flake of Arizona

Foto: Jeff Flake source AP

Why he could vote no: Flake has long been critical of Trump and has declined to run for reelection, giving Republican leaders little ammunition to pressure the Arizona senator. Flake raised concerns regarding the potential deficit impact of the bill after it was released and has maintained criticisms since.

John McCain of Arizona

Foto: source Associated Press/J. Scott Applewhite

Why he could vote no: McCain was an opponent of the Obamacare repeal push due to its hasty process in the Senate, and he called for regular order on any legislation going forward. While McCain praised the Senate Finance Committee’s markup of the TCJA, it is unclear if that will be enough to satisfy his call for bipartisanship. McCain voted against tax cuts under President George W. Bush due to concerns about its effect on the deficit.

Bob Corker of Tennessee

Foto: source Chip Somodevilla/Getty Images

Why he could vote no: Similar to Flake, Corker has been part of a public feud with Trump and is also set to leave the Senate after 2018. And like Flake, Corker is worried about the debt and deficit effects of the bill.

Steve Daines of Montana

Foto: Sen. Steve Daines source Susan Walsh/AP Images

Why he could vote no: Daines reportedly has similar concerns to Johnson over the tax bill’s treatment of pass-through businesses. The Montana senator tweeted on Monday, however, that he spoke with Trump to discuss his concerns. But he’s a “no” on the current version.

James Lankford of Oklahoma

Foto: Sen. Steve Lankford source Manuel Balce Ceneta/AP Images

Why he could vote no: Lankford raised serious concerns about the TCJA’s projected additions to the federal deficit. The Oklahoma senator even told reporters Monday that he was working with leadership on a “backstop” in case the tax receipts crash and the bill produces little of the economic growth Republican leaders have promised.

Lisa Murkowski of Alaska

Foto: Alaska Senator Lisa Murkowski source Reuters

Why she could vote no: Murkowski previously helped bring down the GOP push to repeal and replace Obamacare, so she has a track record of voting against the party-line on contentious issues.

While the Alaska senator did release an op-ed saying she supported the repeal of the Obamacare’s individual mandate in the tax bill, she also called for a simultaneous passage of the Alexander-Murray Obamacare stabilization package. Additionally, her opinion piece did not expressly endorse the bill. While the path to get Murkowski’s vote appears easier than on other pieces of legislation, it is not guaranteed.

Other wild cards…

Foto: source Drew Angerer/Getty Images

Some other Republican members have expressed concerns about various aspects of the TCJA. For instance, Sen. Marco Rubio has called for a more generous child tax credit that is fully refundable. It is unclear, however, whether a lack of change would sink his support.

Similarly, Jerry Moran of Kansas has expressed worry over the repeal of Obamacare’s individual mandate and his own state of Kansas’ failed history with tax cuts. GOP leaders are confident they will keep him on board, however.