(Adds White House comments paragraphs 18-19, details on timing of bill in paragraphs 13-14)
WASHINGTON, Dec 20 (Reuters) - The Republican-controlled U.S. House of Representatives on Wednesday was expected to give final approval to a sweeping tax bill and send it to President Donald Trump to sign into law, sealing his first major legislative victory in office.
In the largest overhaul of the U.S. tax code in 30 years, Republicans in mere weeks steamrolled over the opposition of Democrats to slash taxes for corporations and the wealthy, while offering mixed, temporary tax relief to middle-class Americans.
The Senate approved the bill in the wee hours of Wednesday morning on a 51-48 vote but had to send it back to the House, which had passed it on Tuesday, for another vote due to a procedural foul-up that embarrassed Republicans, but was not expected to change the outcome.
The second House vote was expected about noon on Wednesday.
In an overnight post on Twitter Trump said he would hold a news conference at 1 p.m. EST (1800 GMT) if the House approves it. White House spokeswoman Sarah Sanders said Trump would hold a tax-related event at the White House with U.S. lawmakers at 3 p.m. (2000 GMT) but not to sign the bill, which still faces procedural hurdles.
After Republicans rushed to get the bill passed before the end of the year, White House economic adviser Gary Cohn said the timing of signing it into law depends on whether automatic spending cuts triggered by the bill could be waived. If so, the president will sign it before the end of the year, he said.
The sprawling, debt-financed legislation cuts the U.S. corporate income tax rate to 21 percent from 35 percent, gives other business owners a new 20 percent deduction on business income and reshapes how the government taxes multinational corporations along the lines the country's largest businesses have recommended for years.
Millions of Americans would stop itemizing deductions under the bill, putting tax breaks that incentivize home ownership and charitable donations out of their reach, but also making tax returns somewhat simpler and shorter.
TOP TAX RATE REDUCED
It keeps the present number of tax brackets but adjusts many of the rates and income levels for each one. The top tax rate for high earners is reduced. The estate tax on inheritances is changed so far fewer people will pay.
In two provisions added on to secure needed Republican votes, the legislation also repeals part of the Obamacare health system and allows oil drilling in Alaska's Arctic National Wildlife Refuge.
Democrats have railed against the legislation as a giveaway to the wealthy and the business community that will widen the income gap between rich and poor, while adding $1.5 trillion over the next decade to the $20 trillion national debt, which Trump promised in 2016 he would eliminate as president.
Democratic Senator Chris Van Hollen said the bill "will harm millions of middle-class families ... It contains huge, permanent giveaways for big banks and corporations, and asks our children, millions of working Americans and senior citizens, and future generations to pay the price."
A few Republicans, whose party was once defined by its fiscal hawkishness, have protested the deficit-spending encompassed in the bill. But most of them have voted for it anyway, saying it would help businesses and individuals, while boosting an already expanding economy they see as not growing fast enough.
"Weve had two quarters in a row of 3 percent growth," Senate Republican leader Mitch McConnell said after the chamber's vote. "The stock market is up. Optimism is high. Coupled with this tax reform, America is ready to start performing as it should have for a number of years."
Despite Trump administration promises that the tax overhaul would focus on the middle class and not cut taxes for the rich, the nonpartisan Tax Policy Center, a think tank in Washington, estimated middle-income households would see an average tax cut of $900 next year under the bill, while the wealthiest 1 percent of Americans would see an average cut of $51,000.
TAKES EFFECT IN EARLY '18
The bill, once finalized, would take effect in early 2018 with taxpayers likely to see the first changes to their paycheck tax withholdings in February.
Most households will not see the full effect of the tax plan on their income until they file their 2018 taxes in early 2019.
The prospect of a Republican victory was tinged with embarrassment. House lawmakers initially voted 227-203, largely along party lines, to approve the bill on Tuesday afternoon.
The measure went to the Senate, where the parliamentarian ruled three minor provisions in violation of an arcane Senate rule. To proceed, the Senate deleted the three provisions and then approved the bill.
Because the House and Senate must approve the same legislation before Trump can sign it into law, the Senate's late Tuesday vote sent the bill back to the House.
Democrats pounced on the procedural mistake as evidence of the hurried, often secretive process used by Republicans in developing the bill. Ignoring Democrats and much of their own rank-and-file, Republican congressional leaders and White House officials drafted the bill behind closed doors, unveiling it on Sept. 27.
No public hearings were held and numerous narrow amendments favored by lobbyists were added late in the process, tilting the package more toward businesses and the wealthy.
"After only a few months of frantic backroom negotiations by only one party, we are left with a product as sloppy and as partisan as the process used to draft it ... What a disgrace," Senate Democratic Leader Chuck Schumer said on the Senate floor.
U.S. House Speaker Paul Ryan defended the bill in television interviews on Wednesday morning, saying support would grow for after it passes and Americans felt relief.
"I think minds are going to change," Ryan said on ABC's "Good Morning America" program.
(Reporting by David Morgan and Amanda Becker; Additional reporting by Richard Cowan, Roberta Rampton, Gina Chon and Susan Heavey; Editing by Jeffrey Benkoe and Bill Trott)