A monthlong, double-digit slide in the shares of Firstar Corp. and its merger partner, U.S. Bancorp, is casting doubt on the planned combination and has even dredged up rumors that the latter passed up a lucrative cash takeover bid earlier this year.

Shares of the two companies have skidded virtually in lockstep, down nearly 13% since the deal was announced Oct. 4. The pain may be worse for U.S. Bancorp shareholders, however. Their stock has dropped 36% since last year after a string of disappointing profit reports and the resignations of key senior executives.

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