US senators on Saturday (Sunday local time) began debating 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 $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 $3 trillion to the country's debt.

The Senate formally opened debate on the bill late 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.

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.

Divisive cuts

Republicans are scrambling to offset the $4.5 trillion cost of Trump's tax relief, with many of the proposed cuts to come from decimating funding for Medicaid, the health insurance program for low-income Americans.

Republicans are split on the Medicaid cuts, which will threaten scores of rural hospitals and lead to an estimated 8.6 million Americans being deprived of health care.

The spending plan would also roll back many of the tax incentives for renewable energy that were put in place under Trump's predecessor Joe Biden.

On Saturday, key Trump ally Elon Musk -- with whom the president had a public falling out this month over his criticism of the bill -- called the current proposal "utterly insane and destructive."

"It gives handouts to industries of the past while severely damaging industries of the future," said Musk, who is the world's richest person, and owns electric vehicle company Tesla and space flight firm SpaceX, among others.

Independent analysis also shows that the bill would pave the way for a historic redistribution of wealth from the poorest 10 percent of Americans to the richest.

The bill is unpopular across multiple demographic, age and income groups, according to extensive recent polling. 

Although the House has already passed its own version, both chambers have to agree on the same text before it can be signed into law.