Even in the best of times, carrying a balance on your credit card is a risky — and costly — proposition. These days, it can be downright foolish, at least if there’s a chance you might miss a payment or two.

Millions of cardholders have recently received letters from the likes of Citibank, Bank of America Corp., Wells Fargo & Co. and American Express Co. notifying them that their interest rates are going up, in some cases to 30% if a single payment is missed.

JPMorgan Chase & Co., the nation’s largest issuer of plastic, has begun charging hundreds of thousands of cardholders a $10 monthly fee for having carried large balances for more than a couple years.

Why? In part it’s because default rates are rising and banks are dealing with additional risk. But lawmakers and consumer advocates say the higher rates also reflect banks’ massive losses from betting wrong on the housing boom, and they’re basically sticking credit card customers with the tab.

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