(NAHB corrects size of tax credit it has been seeking to 12 percent from $5,500 in paragraph 5)

By David Morgan

WASHINGTON, Oct 29 (Reuters) - President Donald Trump's plan for overhauling the U.S. tax system faced growing opposition from interest groups on Sunday, as Republicans prepare to unveil sweeping legislation that could eliminate some of the most popular tax breaks to help pay for lower taxes.

Republicans who control the U.S. House of Representatives will not reveal their bill until Wednesday. But the National Association of Home Builders, a powerful housing industry trade group, is already vowing to defeat it over a change for home mortgage deductions, while Republican leaders try to head off opposition to possible changes to individual retirement savings and state and local tax payments.

Trump and Republicans have vowed to enact tax reform this year for the first time since 1986. But the plan to deliver up to $6 trillion in tax cuts for businesses and individuals faces challenges even from rank-and-file House Republicans.

House and Senate Republicans are on a fast-track to pass separate tax bills before the Nov. 23 U.S. Thanksgiving holiday, iron out differences in December, send a final version to Trump's desk before January and ultimately hand the president his first major legislative victory. Analysts say there is a good chance the tax overhaul will be delayed until next year.

The NAHB, which boasts 130,000 member firms employing 9 million workers, says the bill would harm U.S. home prices by marginalizing the value of mortgage interest deductions as an incentive for buying homes. The trade group wants legislation to offer a tax credit equaling 12 percent of mortgage interest and property tax payments but says it was rebuffed by House Republican leaders.

"We're opposed to the tax bill without the tax credit in there, and we'll be working very aggressively to see it defeated," NAHB chief executive Jerry Howard told Reuters.

Republicans warned that the Trump tax plan is entering a new and difficult phase as lobbyists ramp up pressure on lawmakers to spare their pet tax breaks.

"When groups start rallying against things and they succeed, everything starts unraveling," Senator Bob Corker, a leading Republican fiscal hawk, told CBS' Face the Nation.

ANXIETY IN HIGH-TAX STATES

One of the biggest challenges involves a proposal to eliminate the federal deduction for state and local taxes (SALT), which analysts say would hit upper middle-class families in high income tax states such as New York, New Jersey and California. The states are home to enough House Republicans to stymie legislation.