Declines of more than 20% in regional bank stocks may be a major sign that the rally in the Standard & Poor's 500 index is about to fizzle.

Smaller lenders have lost 24% of their market value since climbing to a four-month peak on May 8. Concern that mortgage rates, credit losses and foreclosures are rising spurred retreats in those companies expected to benefit most from $12.8 trillion of government stimulus spending.

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