After an official 20 hours of debate, the Republican-controlled Senate was expected to begin a potentially chaotic "vote-a-rama" on amendments from Republicans and Democrats before moving to a final vote late on Thursday or early on Friday.
U.S. financial markets have rallied on optimism that the measure could pass, a sentiment shared by outside conservative groups that hope to see the first major overhaul of the U.S. tax code since 1986, when Republican Ronald Reagan was president.
"It's the most unified effort I've seen on any issue in many years," said Tim Phillips, president of Americans for Prosperity, a group aligned with billionaire industrialists Charles and David Koch.
A Republican push to overturn Obamacare ended in a humiliating failure in the Senate earlier this year, and President Donald Trump and his Republican allies have since been under mounting pressure to enact a package of tax cuts for businesses and individuals before January, giving them their first major legislative victory.
Republicans acknowledge that failure to pass a tax bill could jeopardize their control of the Senate and House of Representatives in next year's congressional elections.
Democrats say the Republican tax plan is a giveaway to corporations and the wealthy at the expense of working Americans.
The House approved its own tax bill on Nov. 16. If passed this week, the Senate legislation would need to be reconciled with the House version before a final bill could be sent to Trump.
As an initial action on Thursday, Senate Republicans were expected to take a procedural vote that would formally replace the House bill with their own legislation.
While campaign donors are strongly behind the push for tax cuts, the American public is sharply divided.
Among Americans aware of the Republican tax plan, 49 percent said they were opposed, up from 41 percent in October, according to a Nov. 23-27 Reuters/Ipsos poll released on Wednesday. The latest online poll of 1,257 adults found 29 percent supporting the plan and 22 percent saying they "don't know."
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