Do Senate Republican leaders and the White House have the 50 GOP votes to pass a fiscal 2018 budget resolution that is linked to possible tax cuts? “Who knows?” says President Donald Trump, the leader of the party.

The Senate is poised to begin a marathon series of votes Thursday that will culminate with a final vote on a GOP-crafted fiscal 2018 budget resolution. Republicans who helped derail recent major legislation, like Sen. Susan Collins of Maine, this week have expressed confidence that ample votes will be there.

But Trump’s not so sure.

“I think we have the votes, but who knows?” the president tweeted Thursday morning.

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

Trump’s skepticism comes three days after he stood in the Rose Garden with Senate Majority Leader Mitch McConnell and said this of the Kentucky Republican’s ability to pass the coming tax bill: “This man is going to get it done.”

The tweet reflects the president’s lingering anger at Sen. John McCain, R-Ariz., who cast the vote that sunk a GOP health care overhaul bill to the surprise of Republican leaders, White House legislative affairs staff, and the president himself. He often brings up McCain’s move, describing it as a betrayal of the Republican Party and its seven-year-old promise to repeal and replace Barack Obama’s 2010 health care law.

Watch: Trump on McConnell: We’re ‘Closer Than Ever Before’

For Trump and Republican senators, there is a lot riding on the final vote late Thursday night or early Friday morning in the Senate chamber. That’s because the budget resolution contains special rules — known in Washington as “reconciliation” — that would allow a follow-on tax measure to pass the chamber with just 51 votes.

That’s important, as no Democrat has announced support for the tax overhaul bill Republicans and Trump administration officials are crafting. Without a 51-vote rule, a tax measure would need 60 votes to pass — meaning a half dozen or more Democrats would be needed.

With the reconciliation rule, 50 GOP senators plus Vice President Mike Pence’s 51st tie-breaking vote would be enough.

To that end, in his tweet the president called the budget resolution the “first step toward massive tax cuts.” Sen. Lindsey Graham of South Carolina and other Republicans are warning their colleagues that a failure on taxes, after the fits and starts on health care, could spell doom for their party in the 2018 midterm elections.