Although a bill aimed at reviving home sales and curtailing foreclosures is about to become law, some of its provisions are proving a drag for the nation's large home builders.

Despite a rally Tuesday, the Dow Jones Wilshire U.S. Home Construction Index, which tracks the stocks of major builders, has fallen about 8.5% since President George W. Bush indicated last week that he wouldn't veto the bill that has been approved by Congress.

There have been months of intense lobbying by the building industry, but analysts say the legislation is a mixed bag for the new-home market. On the bright side, the bill shores up mortgage giants Fannie Mae and Freddie Mac, which should help restore some confidence in the mortgage market. It also provides a $7,500 tax credit to stimulate demand among first-time home buyers.

But for the builders, the bill's elimination of seller-funded down-payment assistance on mortgages backed by the Federal Housing Administration is a big loss -- one that could eliminate as many as one in 10 home buyers from the market, according to an analyst.

Starting in October, buyers using FHA loans can no longer accept down-payment "gifts" that are ultimately funded by the home seller, often a builder. Currently, the FHA allows a nonprofit group to gift the down-payment to the buyer. The nonprofit group is then reimbursed by the builder -- a practice the housing bill would stop.

Seller-funded down-payment assistance, which essentially allows buyers to purchase homes with little or no money down, has been filling the void left when the subprime-mortgage market all but vanished in 2007.

FHA officials have said loans that include the down-payment gifts are incurring higher default rates than FHA loans without the gifts. Builders say the gifts make homeownership possible for low-income buyers who have been unable to save money for a down payment.

Miami-based Lennar Corp. used down-payment assistance on 33% of the mortgages it originated in the second quarter, while Ryland Group Inc. said 18% to 20% of its buyers used down-payment assistance during the first half of the year.

"We believe a material portion of these people won't be able to find alternative mortgage-financing options," says Michael Rehaut, a housing analyst at J.P. Morgan. "This likely will result in an additional segment of demand leaving the market."

The elimination of down-payment gifts could bar as much as 10% of the nation's home-buyer pool and as many as 25% of buyers in lower-priced markets, such as Texas, where the gifts are more prevalent, according to housing researcher Zelman & Associates.

Complicating matters further for the builders, the housing bill would increase the down-payment requirement on FHA loans to 3.5% from 3%. Previous versions of the measure had lowered the down payment to 1.5%.

"There will undoubtedly be some impact, but we believe the buyers will adjust and the market will adjust," says Tim Eller, the chief executive of Centex Corp, which said that 25% of its sales in its fiscal year ended March 31 involved down-payment assistance.

In the absence of down-payment gifts, Mr. Eller says, his company can put buyers on a savings plan. In addition, he says that not everyone who uses down-payment assistance is necessarily unable to come up with a down payment.

Mr. Eller also is hopeful that a tax-credit provision in the legislation will help stimulate demand. "We know that this is no silver bullet, but it will have some stimulative impact," he say.

The tax credit mirrors a similar program in the 1970s that Mr. Eller says allowed him to buy his first home, a condominium outside Chicago. But unlike the credit Mr. Eller used in 1974, the credit proposed in the current housing bill has to be paid back over 15 years, making it more like an interest-free loan.

And a home buyer would typically collect a credit after they file their taxes. That may not help buyers who need a cash infusion -- like a down-payment gift -- at the time of the sale.

It is possible that builders could use their mortgage businesses to lend the tax credit to buyers at the house closing, and then be reimbursed after the buyer files their taxes. But that puts the builder at risk if the buyers default on these loans -- a risk the builder's mortgage units aren't necessarily designed to deal with.

"The primary purpose of the builders' mortgage subsidiaries is to get mortgage origination fees," J.P. Morgan's Mr. Rehaut says. "They don't want to take on default risks."

On a brighter note, builders say the housing bill could boost higher-end sales by raising the conforming-loan limits on Fannie- and Freddie-guaranteed loans and FHA loans to a maximum of $625,000 in some high-priced areas.

But many of those higher-end sales will depend on whether buyers can sell their current homes, often to first-time home buyers, which is why builders say the tax credit will help the overall market.

"First time home buyers can help bring back stabilization" in the market, Centex's Mr. Eller says. "It's a very large cohort, and they can have a big impact."

Write to Michael Corkery at michael.corkery@wsj.com

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