WASHINGTON — Just when consumers and the U.S. economy need banks to lend more freely, the mortgage industry is making it harder to borrow — even for those with good credit.

Mortgage insurers, whose backing is required for borrowers who can't afford the traditional 20 percent down payment on a home, have already flagged nearly a quarter of the nation's ZIP codes where they refuse to insure some home loans.

The entire states of California, Florida, Arizona, Michigan, Ohio and Nevada — which have seen the highest foreclosure rates and the worst price declines — are blackballed on some mortgage insurers' lists. Hawai'i is not categorized as a high-risk area, according to data provided by PMI Inc., a national mortgage insurance company.

Banks that have lost billions because of bad bets during the housing boom are now reverting to strict lending standards not seen in nearly 20 years, according to industry data and interviews with lenders.

For new homebuyers and those seeking to refinance, it can mean higher down payments and a higher bar for credit scores, among other requirements.

The reluctance to extend credit comes despite a flurry of government initiatives, including steady interest rate cuts by the Federal Reserve, intended to make it easier for would-be borrowers and those facing interest-rate resets on their mortgages.

Lenders' growing leeriness threatens to dampen sellers' already soggy prospects for the spring home-buying season — and that means more pain for the battered housing sector and the broader economy.

In recent weeks, mortgage insurers have flagged more than 9,600 ZIP codes in at least 34 states where they won't insure certain types of home loans — those for investment properties or second homes, those with riskier adjustable-rate or interest-only mortgages, or for buyers making down payments of less than 3 percent.

The stinginess of banks is showing up in home loan statistics: The value of all new mortgages plummeted to $450 billion in the fourth quarter of 2007, down 38 percent from a year earlier, according to trade publication Inside Mortgage Finance.