WASHINGTON – President Donald Trump’s tax cut plan will confront a major legislative hurdle next week as Senate Republicans face a cliffhanger vote on their budget resolution.

Republicans control only 52 votes in the 100-seat Senate, and they can afford to lose only two Republican lawmakers to pass the budget resolution – which would give the party the room it needs in the budget to pass tax reform on a party-line vote.

Many Senate Republican aides think that Sen. Rand Paul, R-Ky., will oppose the budget resolution because it will not commit to eliminating the deficit over 10 years, and the votes of other Senate Republicans remain unclear.

“It’s not something they are taking for granted,” said Douglas Holtz-Eakin, a conservative economist who is president of the American Action Forum, referring to Senate Republican leaders. “They are worried.”

Just weeks ago, Senate Republicans were touting progress on the budget deal as a sign of their party’s ability to unify, after party deficit hawks found a compromise with those pushing for the large, supply-side tax cuts to move the bill through committee. But now the budget’s prospects in the broader Senate have again become clouded.

Sen. Thad Cochran, R-Miss., is still recovering from health problems and has not voted in the Senate since mid-September. He is expected back next week but his timetable could not be confirmed.

Sen. John McCain, R-Ariz., has voiced support for the Republican tax cut effort, but he has also not committed to backing the budget resolution.

In the past, McCain has sought assurances that the government’s budget would include more money for the military, something the current Senate budget does not do.

And Sen. Susan Collins, R-Maine, has shown a willingness to buck her party this year, opposing a repeal of the Affordable Care Act. She voted in favor of GOP-driven tax cuts in 2003, but she is also expected to announce Friday whether she will run for governor next year, a decision that could change her approach to legislating for the rest of the year.

Spokespeople for McCain, Collins and Paul declined to say whether the lawmakers would vote for the measure next week.

A spokesman for Cochran didn’t respond to a request for comment.

The House and the Senate must pass matching budget resolutions to proceed on a tax bill that can pass with only 50 votes in the Senate, assuming they also will have the support of Vice President Mike Pence, who can cast a tiebreaker vote.

If they don’t pass matching budget resolutions, then Senate bills need 60 votes to clear procedural hurdles, something that could prove very difficult to achieve if Democrats unify in opposition to the plan.

A number of Democrats have criticized the tax cut framework advanced by the White House and Republican leaders, alleging that it would amount to a huge tax cut for the wealthy, something that Paul has said he worries could be true.

One pivotal vote that Republican leaders appeared to have secured last week was that of Sen. Bob Corker, R-Tenn., who supported the budget resolution during a Senate Budget Committee vote despite misgivings about its potential effect on the debt.

The budget resolution would essentially allow the tax cut plan to add $1.5 trillion to the debt over 10 years, and Corker has said he wants assurances that it would actually reduce the deficit, not increase it. Republican leaders were working to make Corker feel more comfortable with the approach, but Corker and President Trump got into a war of words in recent days. If Corker decides to buck GOP leaders on the Senate budget resolution vote, it could imperil the entire tax plan.

Asked about the feud between Corker and Trump on Monday, Senate Majority Leader Mitch McConnell, R-Ky., said, “Sen. Corker is a valuable member of the Senate Republican caucus and he’s also on the budget committee and a particularly important player as we move to the floor on the budget next week,” according to the Associated Press.

Asked whether his spat with Corker could damage the chances of passing the tax cut plan, Trump on Tuesday said, “I don’t think so, no. I don’t think so at all. I think we’re well on our way.”

The White House and Republican lawmakers are under a tremendous amount of pressure to pass the tax cut plan, having come up short on other priorities earlier in the year. The House already has passed a budget resolution that is very different from the one Senate lawmakers will consider next week, but leaders have expressed confidence that they can work out their differences.

But before they can negotiate with the House, the Senate must pass its version of the budget resolution, which won’t be determined until the vote next week.