The Obama administration on Tuesday will launch its most ambitious effort at reducing mortgage balances for homeowners who owe more than their homes are worth.

Officials say between 500,000 and 1.5 million so-called underwater loans could be modified through the program, the first initiative to target homeowners who are current on their mortgage payments but are at risk of default because they have no equity in their homes. Some experts are warning, however, that the same knots that tied up prior initiatives could do so again.

Under the new "short refinance" program, banks and other creditors that write down mortgages to less than the value of the property can essentially hand off the reduced loan to the government. The process involves refinancing borrowers into loans backed by the Federal Housing Administration.

While the program puts taxpayers at risk—officials estimate one in five loans in the program could default—the government has set aside $14 billion previously earmarked for housing aid from the Troubled Asset Relief Program to cover losses.

The new program, which was announced in March, is starting as the housing market shows signs of renewed trouble and as the Obama administration's signature Home Affordable Modification Program, or HAMP, falls short of its goals of helping three million homeowners. Half of the 1.3 million borrowers that enrolled in temporary loan modifications have fallen out of HAMP because they didn't qualify. Only one-third has received permanent modifications.

The initiative also comes as mortgage rates fall to their lowest levels in more than 50 years. Average rates on 30-year fixed-rate loans dropped to 4.43% last week, down from 4.55% during the previous week, according to a survey published Wednesday by the Mortgage Bankers Association.

One of the biggest dangers facing the housing market is the glut of underwater homeowners who could default if their personal finances or home prices worsen. About 11 million borrowers, or 23% households with a mortgage, were underwater as of June 30, according to CoreLogic Inc.

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