The US Senate has approved a sweeping tax overhaul, moving Republicans and President Donald Trump a major step closer to their goal of slashing taxes for businesses and the rich while offering everyday Americans a mixed bag of changes.

In what would be the largest US tax overhaul since the 1980s, Republicans want to add $US1.4 trillion ($A2.0 trillion) over 10 years to the $US20 trillion national debt to finance changes that they say would further boost an already growing economy.

US stock markets have rallied for months in hopes Washington would provide significant tax cuts for corporations.

Following the 51-49 vote on Saturday, talks will begin, likely next week, between the Senate and the House of Representatives, which has already approved its own tax bill. The two chambers must craft a single bill to send to Trump to sign into law.

The president wants that to happen before the end of the year, allowing him and his Republicans to score their first major legislative achievement of 2017, despite controlling the White House, the Senate and the House since he took office in January.

A tax overhaul is seen by Republicans as crucial to their prospects in the November 2018 mid-term election campaigns when they will have to defend their majorities in Congress.

In a legislative battle that moved so fast a final draft of the bill was unavailable to the public until just hours before the vote, Democrats slammed the measure as a give-away to businesses and the rich financed with billions in taxpayer debt.

The framework for both the Senate and House bills was developed in secret over a few months by a half-dozen Republican congressional leaders and Trump advisers, with little input from the party’s rank-and-file and none from Democrats.

The Senate approved their bill in the wee hours, with Democrats complaining that last-minute amendments circulated among senators were poorly drafted and vulnerable to being gamed later by lawyers and accountants in the tax avoidance industry.

“The Republicans have managed to take a bad bill and make it worse,” said Senate Democratic leader Chuck Schumer.

“Under the cover of darkness and with the aid of haste, a flurry of last-minute changes will stuff even more money into the pockets of the wealthy and the biggest corporations.”

No Democrats voted for the bill, but they lacked the votes to block it because Republicans hold a 52-48 Senate majority.

Republican Senator Bill Cassidy praised the bill and said, “Working families and middle-income families across the nation will be better off. Families who over the last eight years have not done well will begin to do better.”

-AAP