Flailing automakers hoping to jump-start sales could face yet another roadblock: deteriorating conditions in an important funding source for auto loans.

Bonds backed by such loans are under severe stress as high gasoline prices and a slowing economy cause borrowers to fall behind on payments. Analysts expect the bonds issued last year to be the worst-performing ones of the past decade.

Reduced demand for such bonds will make it tougher for consumers to secure affordable financing.

"In the auto sector, lack of credit availability means it will be harder to buy cars," Chris Flanagan, the head of global structured finance research at JPMorgan Chase & Co., said at an automotive conference last week.

Tepid demand has caused a jump in risk premiums and a slowdown in new bond deals backed by auto loans as funding has become more expensive for U.S. automakers' finance arms.

Barclays Capital estimates that new offerings from Ford Motor Co., General Motors Corp., and Chrysler LLC would cost 50 basis points more if they chose to tap the market. Kavyan Darouian, a consumer asset-backed securities analyst at Barclays Capital in New York, noted that there have not been any offerings in a month.

Existing bonds from the three automakers have been trading at a margin 100 to 200 basis points wider than those from Japanese automakers' securities, Mr. Darouian said.

"There is a lot of credit tiering in the market," as has always been the case, but it is much more pronounced now, he said.

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