US senators debated into the early hours of Sunday Donald Trump's "big beautiful" spending bill, a hugely divisive proposal that would deliver key parts of the US president's domestic agenda while making massive cuts to social welfare programs.

Trump is hoping to seal his legacy with the "One Big Beautiful Bill," which would extend his expiring first-term tax cuts at a cost of US$4.5 trillion and beef up border security.

But Republicans eyeing 2026 midterm congressional elections are divided over the package, which would strip health care from millions of the poorest Americans and add more than US$3 trillion to the country's debt.

The Senate formally opened debate on the bill late on Saturday, after Republican holdouts delayed what should have been a procedural vote -- drawing Trump's ire on social media.

Senators narrowly passed the motion to begin debate, 51-49, hours after the vote was first called, with Vice President JD Vance joining negotiations with holdouts from his own party.

Ultimately, two Republican senators joined 47 Democrats in voting "nay" on opening debate.

Trump has pushed his party to get the bill passed and on his desk for him to sign into law by July 4, the United States' independence day.

"Tonight we saw a GREAT VICTORY in the Senate," Trump wrote on his Truth Social platform after the vote to begin debate.

"Republicans must remember that they are fighting against a very evil, corrupt and, in many ways, incompetent (Policywise!) group of people, who would rather see our Country 'go down in flames' than do the right thing," he said in an earlier post.

Democrats are bitterly opposed to the legislation and Trump's agenda, and have vowed to hold up the debate. They began by insisting that the entirety of the bill be read aloud to the chamber before the debate commences.

The bill is roughly 1,000 pages long and will take an estimated 15 hours to read.

"Republicans won't tell America what's in the bill," said Senate Democratic Leader Chuck Schumer. "So Democrats are forcing it to be read start to finish on the floor. We will be here all night if that's what it takes to read it."

If passed in the Senate, the bill would go back to the House for approval, where Republicans can only afford to lose a handful of votes -- and are facing stiff opposition from within their own ranks. (AFP)