President Trump plans to meet Tuesday with Senate Republicans in a crucial week for his presidency and his tax plan that could decide whether he ends his first year with a major legislative achievement.

Last-minute threats to the tax cuts that Trump has promised since early in his long-shot presidential campaign are looming among Senate Republicans. Concerns center on the taxation of partnerships, limited liabilities and other companies; on the overall cost of the tax bill itself; and on a familiar Republican stumbling block – Obamacare.

“It’s not going to be easy,” Senate Finance Committee Chairman Orrin Hatch of Utah said Monday. “This is going to be a tough, tough time.”

Senate Republican leaders aren’t counting on any Democrats for a floor vote that could come Thursday or Friday – but that means they can afford to lose only two members of their own party. Here are the eight Republican senators most likely to decide the tax bill’s fate.

BOB CORKER, JEFF FLAKE AND JAMES LANKFORD

Corker, the retiring Tennessee Republican has staked a hard line against letting tax legislation add to federal deficits – saying that a single penny of new deficits would lose his vote. It turns out the Senate bill would add $1.4 trillion to the deficit over 10 years – at least before accounting for any economic growth – according to a Congressional Budget Office report released Sunday.

You could take the entire individual portion of this bill, throw it in the trash, "put it in the incinerator, and I would be thrilled" – @SenBobCorker. on @CNBC now

— Carl Quintanilla (@carlquintanilla) November 28, 2017

The bill’s supporters say it’ll boost economic growth enough to cover that shortfall, but Corker says he’s not satisfied. He wants a backstop mechanism – essentially a tax-increase trigger that would raise revenue in case the promised growth doesn’t result. Arizona’s Flake and Oklahoma’s Lankford also support that kind of trigger.

Sen. James Lankford, Republican from Oklahoma.

“Every economist is guessing,” Lankford said on Monday. “We should build in the ‘what if’ – what if this doesn’t work?’ If the revenue is not coming in, should the rates change?”

Corker is a member of the Senate Budget Committee, which meets Tuesday afternoon to decide whether to send the tax bill to the Senate floor. Republicans have a one-vote margin on the panel and cannot afford any defections. Corker said Monday he may vote no if tax writers – who he said would be working overnight on a provision – can’t provide enough deficit safeguards.

“We’re working very closely with the administration and also some members of the Finance Committee to design a trigger or backstop that in the event the revenues are not there, there’s a way to recoup them,” he said.

Some senators want nothing to do with a trigger. “I think the problem with triggers is they create uncertainty about what the tax rate is” since it’s subject to change, said Republican Pat Toomey of Pennsylvania, during a Bloomberg Television interview. Resulting hesitancy among investors could make the predictions of too little growth “a self-fulfilling prophecy,” he said.

Corker disagrees. “There are ways of doing this that you don’t damage the aspects that are very pro-growth,” he said.

Flake and Corker have two things in common beyond deficit concerns: Both are retiring after 2018, and both have sparred extensively with Trump. So it’s unlikely that any presidential salesmanship would sway either one.

“He’ll be a NO on tax cuts because his political career anyway is ‘toast,’ ” Trump said on Twitter this month about Flake, whom he called “Jeff Flake(y).”

RON JOHNSON AND STEVE DAINES

Johnson became the first Republican senator to come out against the bill, and his vote remains up in the air. His concern? The legislation gives an advantage to large corporations at the expense of “pass-through” businesses, like the plastics company he used to run before his election to the Senate in 2010.

The Wisconsin senator is also a member of the Budget Committee, and he too wants his concern addressed by the time it meets. “I’m not exactly sure what’s going to happen in committee, and we’re working diligently to fix the problem,” he told reporters Monday. “If we develop a fix prior to committee, I’ll probably support it but if we don’t, I’ll vote against it.”

The Senate bill cuts the corporate rate to 20 percent, while creating a special deduction for business income from pass-through entities that would leave many owners paying an effective top rate of more than 30 percent. Johnson wants to give those owners a larger deduction, and he said many have called to encourage him to stand firm.

Daines, a Montana Republican, joined Johnson Monday in opposing the bill as written, and for the same reason. An aide said Daines remains optimistic the legislation will change enough to win his vote.

A deal had yet to emerge Monday night. “We’re still negotiating; let’s put it that way,” said South Dakota Sen. John Thune, the chamber’s third-ranking Republican leader, after an evening meeting of the Finance panel.

Daines says the more generous tax cut that he and Johnson favor would cost about $60 billion more over 10 years – and they have a proposal to pay for it: Eliminate the income-tax deduction that corporations can take for the state and local taxes they pay.

A similar provision to eliminate so-called SALT deductions for individuals has stirred opposition among Republican House members in high-tax states; adding corporations to the list may engender still more resistance.

JOHN McCAIN

Nobody is taking McCain’s vote for granted after he shocked the political world by voting against a rushed attempt to demolish the Affordable Care Act this summer.

Sen. John McCain, Republican from Arizona, speaks to members of the media on Nov. 14 at the U.S. Capitol.

He then delivered a speech demanding the Senate return to regular order – hearings, markups, bipartisan input and amendments – for passing major bills. So far, McCain has been warm to the Senate Republicans’ tax effort, but he hasn’t taken an official position on the legislation.

McCain has a mixed record on tax cuts, voting against measures in 2001 and 2003. The top legislative priority of the Arizona Republican, who is 81 and fighting brain cancer, is to boost military spending. His vote on the tax bill could be in peril if he believes the $1.4 trillion in new deficits would put downward pressure on Pentagon funding.

JERRY MORAN

The Kansas Republican is sensitive about the impacts of the bill in the wake of his state’s failed tax-cut experiment that lawmakers there ended this year to escape a fiscal crisis.

“I’m also cognizant of what people saw happen in Kansas,” Moran told constituents over Thanksgiving weekend, as quoted by the Topeka Capital-Journal. “The issue of tax cuts would be easier if you actually had faith that Congress would hold the line on spending. It’s two components. It’s how much revenue you take in and how much money you continue to spend.”

Overall, Moran remained circumspect about the legislation, saying the goal must be to find tax cuts that grow the economy without raising the debt.

SUSAN COLLINS

The moderate from Maine is the only Republican senator from a reliably Democratic-leaning state, and as such she’s always a difficult vote for party leaders. While Collins was initially warm to the tax bill, she has turned sour after party leaders opted to add repeal of the Obamacare individual mandate to help limit deficits.

Collins said it’s a mistake to ax the mandate in tax legislation, fearing that it’ll cause healthy people to drop their coverage and drive premiums higher for others – the same reason she cast a pivotal vote to block an Obamacare repeal bill in July. “I hope that will be dropped,” she said recently.

But eliminating the mandate is estimated to save the federal government more than $300 billion over 10 years – savings that would result from less demand for federal health-coverage subsidies. Some Republican leaders dispute that estimate, even though their tax bill depends on it to stay within budget rules.

Collins has also said she opposes lowering the top individual rate from 39.6 percent to 38.5 percent, as the Senate bill does.

Maine’s other Senator, Independent Angus King who caucuses with the Democrats, came out strongly against the plan on Tuesday, joining Virginia Democrat Tim Kaine in a piece posted to Kaine’s Medium account and online political news site Axios. The senators argue the Senate could line up 70 votes for a more moderate proposal arrived at through regular procedural order. It also reads, in part, “And, we are currently scheduled to debate the bill on the Senate floor this week under a constrained procedure where a Senator offering an amendment is given one minute — one minute! — to debate the amendment before voting. A city council wouldn’t amend the leash law with a process like this.”

My tax reform op-ed with @TimKaine in @axios. Senator Kaine and myself, along with many of our colleagues, want to work with Senate leadership to boost our economy and help the middle class – but this process isn’t the way to do it. Read more: https://t.co/F0QSvgkLm9

— Senator Angus King (@SenAngusKing) November 28, 2017

Bloomberg’s Laura Davison and Kaustuv Basu contributed to this story.