WASHINGTON — The federal debt limit must be increased by May 16 or the Treasury Department will have to take extraordinary measures, such as suspending the sale of state and local government series securities to muni issuers, Treasury Secretary Timothy Geithner said Monday.

“These actions, which have been employed during previous debt-limit impasses, would be exhausted after approximately eight weeks, meaning no headroom to borrow within the limit would be available after about July 8, 2011,” Geithner told House and Senate leaders in a three-page letter.

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