President Trump on Thursday said he thinks there is enough to support in the Senate to pass its 2018 budget resolution, a key step toward tax reform, but added, "who knows?"

"Republicans are going for the big Budget approval today, first step toward massive tax cuts. I think we have the votes, but who knows?" he tweeted.

Republicans are going for the big Budget approval today, first step toward massive tax cuts. I think we have the votes, but who knows? — Donald J. Trump (@realDonaldTrump) October 19, 2017

The Senate takes up the budget on Thursday with a free-wheeling vote-a-rama marathon during which there are fewer restrictions on amendments.

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If Republicans can pass the budget resolution, they can utilize a legislative tool called reconciliation that would allow them to move tax legislation through the Senate on a simple majority vote.

Otherwise, tax reform would need 60 votes, which would make the GOP proposal's passage much less likely, as it would need to pull support from all Republican lawmakers as well as some Democrats.

Trump’s Council of Economic Advisers released a paper on Monday arguing that the GOP proposal to cut the corporate tax rate would help working families by increasing average household incomes by $4,000 to $9,000 annually

“We want to make sure that the middle class is the biggest beneficiary of the tax cuts and tax reform,” Trump told reporters on Monday.

“It's a middle-class bill. That's what we're thinking of. That's what I want,” the president continued.