In California, delinquencies on prime mortgages could increase for years, said Christopher Thornberg, founder of consulting firm Beacon Economics in Los Angeles.
One reason, he said, is that home lenders became so complacent during the housing boom that they did little to qualify borrowers besides having computers check a few facts.
" 'Prime' lost a lot of meaning in the insanity of the last few years," said Thornberg, who was one of the first experts to foresee the housing downturn.
To be sure, the damage has been greatest in subprime mortgages, the high-risk loans tapped heavily during the go-go years by borrowers with the worst credit, the heaviest debt loads or the lowest down payments (and sometimes all three of those).
In August, more than 43% of subprime loans nationally were in foreclosure or at least 60 days late in paying, a rate nearly double that of August 2007, according to First American CoreLogic's LoanPerformance unit, which tracks 82% of all U.S. loans.
But problems with prime loans are increasing as fast or faster. About 7.5% of prime jumbo mortgages -- high-quality home loans too large to be sold to government-backed Fannie Mae and Freddie Mac -- were at least 60 days late or in foreclosure, according to LoanPerformance. That was more than three times the level of a year earlier.
As a result, prime loans account for a larger proportion of foreclosures than they did in August 2007.