– Shiller and other economists worry that prices could fall below the levels of April 2009. That was the lowest point since the peak in July 2006.The Associated Press

NEW YORK – The housing slump isn’t over.

Tax credits and historically low mortgage rates have failed to lift home prices so far this year. Prices fell 0.5 percent in March from February, according to the Standard & Poor’s/Case-Shiller 20-city index released Tuesday.

That marks six straight months of declines — a sign that the housing market is going in reverse.

“It looks a little like a double-dip already,” economist Robert Shiller said in an interview. “There is a very real possibility of some more decline.”

The co-creator of the Case-Shiller index, who predicted in 2005 that the housing bubble would burst, says he worries that home prices rose last year only because of the federal tax credits. That fear is shared by other economists. They note that weak job growth, tight credit and millions more foreclosures ahead will weigh on the home market.

All that is discouraging for homeowners who have seen the value of their largest asset deteriorate sharply over the past three years. Falling home prices tend to curtail consumer spending. And they make it harder for struggling borrowers to refinance into an affordable home loan.

Prices in 13 of the 20 cities tracked by the index fell. Only six metro areas recorded price gains. One, Boston, came in flat.

In the first quarter of 2010, U.S. home prices fell 3.2 percent compared with the fourth quarter.

The numbers are especially disturbing because they show that improved sales due to the tax credits didn’t translate into higher prices, said David M. Blitzer, Chairman of the S&P index committee.

Still, falling home prices haven’t kept many consumers from maintaining their optimism about the economy.

A separate report Tuesday showed consumer confidence rose in May for the third straight month as hopes for job growth improved. The increase in the Conference Board’s Consumer Confidence Index was boosted by consumers’ brighter outlook for the next six months.

In a healthier economy, extraordinarily low mortgage rates would pump up demand for homes. But employers aren’t creating new jobs fast enough and loans are harder to come by for small businesses and individuals.

On Monday, the National Association of Realtors said sales of previously occupied homes rose 7.6 percent in April. But the sales were aided by the government incentives that have now expired. Economists don’t expect the improvements to last.

New buyers were offered a credit worth up to $8,000. Current owners who bought and moved into another home could get a credit for up to $6,500. To receive them, buyers had to have a signed offer by April 30 and must close by the end of June.

Shiller and other economists worry that prices could fall below the levels of April 2009. That was the lowest point since the peak in July 2006.

IHS Global Insight economist Patrick Newport forecasts prices will fall an additional 6 percent to 8 percent and bottom out in the third quarter of next year. Newport said the glut of homes on the market is the main reason. But he’s also worried about the rate of foreclosures.

“When banks foreclose, they sell the properties at deep discounts,” Newport said. “Foreclosures have either peaked in the first quarter or are going to peak soon, but they will remain very high for several years.”

Mortgage delinquencies reached a record high in the first quarter. More than 10 percent of homeowners with a mortgage missed at least one payment from January through March, the Mortgage Bankers Association said last week.

Since 2006, nearly 5 million homes have been lost to foreclosures or other distressed sales, said Mark Zandi, chief economist at Moody’s Analytics. Zandi expects 3 million more to hit the market over the next two years.

Zandi noted that 15 million homeowners still owe more than their homes are worth. And 26 million Americans are either unemployed or underemployed. The underemployed include people who have given up looking for work and part-timers who would prefer to be working full time.

“It’s a lethal mix,” Zandi said.