The US Senate has narrowly approved a tax overhaul, moving Republicans and President Donald Trump a big 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 change to US tax laws since the 1980s, Republicans want to add US$1.4 trillion (AU$2 trillion) over 10 years to the US$20 trillion national debt to finance changes that they say would further boost an already growing economy.
"We are one step closer to delivering MASSIVE tax cuts for working families across America," Mr Trump said in an early-morning tweet.
US stock markets have rallied for months in the hope that Washington would provide significant tax cuts for corporations.
Celebrating their Senate victory, Republican leaders predicted the tax cuts would encourage US companies to invest more and boost economic growth.
"We have an opportunity now to make America more competitive, to keep jobs from being shipped offshore and to provide substantial relief to the middle class," said Mitch McConnell, the Republican leader in the Senate.
The Senate approved their bill in a 51-49 vote with Democrats complaining that last-minute amendments to win over skeptical Republicans 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."
Democratic Senator Elizabeth Warren was given a copy of the 500-page bill just a few hours before the vote.
"That's how [they] make tax policy - late on a Friday night, don't let anybody read it, put this thing through and pay off their donors. This isn't right - this is not how America should be making laws. Shame on the Republicans."
The bill gives a massive tax cut to corporations and repeals parts of Obamacare, with predictions it could leave millions of American's without health insurance.
No Democrats voted for the bill, but they were unable to block it because Republicans hold a 52-48 Senate majority.
Talks will begin, likely next week, between the Senate and the House of Representatives, which has already approved its own tax bill.
Mr Trump 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.
Republicans failed in their efforts to repeal the Obamacare healthcare law over the summer and Mr Trump's presidency has been hit by White House in-fighting and by a federal investigation into possible collusion last year between his election campaign team and Russian officials.
The tax overhaul is seen by Mr Trump and Republicans as crucial to their prospects at mid-term elections in November 2018, 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 proposed tax cuts as a give-away to businesses and the rich financed with billions of dollars 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 Mr Trump advisers, with little input from the party's rank-and-file and none from Democrats.
Under the bill, the corporate tax rate would be permanently slashed to 20 percent from 35 percent, while future foreign profits of US-based firms would be largely exempted from tax - both changes pursued by corporate lobbyists for years.
On the individual side of the tax code, the top tax rate paid by the highest-income earners would be cut slightly.
Reuters / Newshub.
