GMAC LLC, the auto and home lender trying to rescue itself by becoming a bank holding company, scrambled to line up more support for a $38 billion debt swap as some of its biggest investors continued to balk.

Pacific Investment Management Co., which manages the world's biggest bond fund, failed to tender its holdings, according to people familiar with the negotiations. Pimco earlier agreed to participate along with the rest of a bondholder committee that sought better terms, said the people, who declined to be identified because talks were private.

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