* House vote scheduled for 1:30 p.m. EST
* Study says about half of Americans will pay higher taxes
* McConnell: "at the doorstep of an historic opportunity" (Adds quotes from Ryan, McConnell; Flake undecided)
WASHINGTON, Dec 19 (Reuters) - The Republican-controlled U.S. Congress will begin voting on Tuesday on the biggest overhaul of the U.S. tax system in more than 30 years, with little standing in the way of the party's first major legislative triumph under President Donald Trump.
The House of Representatives, which introduced initial tax legislation barely six weeks ago on Nov. 2, was scheduled to vote on Tuesday afternoon. Senate Majority Leader Mitch McConnell said his chamber would vote on Tuesday evening.
With strict party-line votes expected in both chambers, passage appeared all but certain.
"Congress is standing at the doorstep of an historic opportunity," McConnell said on the Senate floor.
Republicans insist the sweeping package of tax cuts for corporations, small businesses and individuals will boost economic and job growth. They also see the measure as key to retaining their majorities in the House and Senate in elections next November.
"This is about expanding opportunities for people who are striving to make the most of their lives," House Speaker Paul Ryan said at a news conference.
Democrats say the legislation will deepen the income gap between rich and poor Americans.
The end-of-year sprint represents a remarkable recovery of Republican fortunes since the middle of this year, when the party's drive to dismantle former Democratic President Barack Obama's Obamacare healthcare law crumbled in the Senate and prospects for a tax overhaul seemed doomed by party infighting.
Lingering doubts about fate of the tax bill all but vanished on Monday after two of the last Senate Republican holdouts, Susan Collins and Mike Lee, agreed to support the legislation.
'NEW GILDED AGE'
Democrats, who unanimously oppose the Republican bill, railed against it as a giveaway to corporations and the wealthy that would add $1.5 trillion to the federal debt over the next decade.
"There are so many rip-offs in this bill that people are going to say this is some kind of new Gilded Age," said Senator Ron Wyden, top Democrat on the Senate Tax Committee.
Some 52 percent of adults oppose the tax plan, while 27 percent support it, according to Reuters/Ipsos polling.
The plan includes a steep tax cut for businesses and temporary tax cuts for individuals. Middle-income households would see an average tax cut of $900 next year, while the wealthiest 1 percent would see an average cut of $51,000, according to the nonpartisan Tax Policy Center.
Roughly half of all Americans would pay higher taxes than they do now after the tax cuts expire in 2025 but the top 1 percent would still pay less, the Tax Policy Center said.
The House, where Republicans hold a 239-193 voting majority, was likely to see a smattering of "no" votes from Republican fiscal hawks and lawmakers from the high-tax states of New York, New Jersey and California who oppose a provision that would scale back a popular deduction for state and local taxes.
Vice President Mike Pence took the precaution of rescheduling a trip to Egypt and Israel for January so he would be on hand this week in case his tie-breaking voting power is needed to ensure Senate passage of the bill.
Republicans, who control the 100-seat Senate by only a 52-48 margin, can afford to lose support from no more than two party lawmakers. Republican Senator Jeff Flake was still undecided on Tuesday. Senator John McCain, who has brain cancer, was spending time with family in Arizona.
The legislation also would repeal a federal fine imposed on Americans under Obamacare for not obtaining health insurance coverage, a change that could undermine the 2010 healthcare law formally known as the Affordable Care Act.
(Additional reporting by Susan Cornwell and Richard Cowan; Writing by Andy Sullivan; Editing by Peter Cooney and Bill Trott)