Rating agencies have not been swift to downgrade monoline insurers, preferring to give the companies sufficient time to address the capital concerns that have been raised. The implications of a significant monoline downgrade for these companies, which rely heavily on their credit rating to underwrite debt insurance, are significant because the insurers provide guarantees for some $2.5 trillion in debt.

MBIA has been the longtime target of activist investor Bill Ackman of Pershing Square Capital, which some five years ago questioned the company's practices and issued a report saying that it should not receive a triple-A rating. Ackman renewed his assault on the company two weeks ago, saying he expected his bets, via his $6 billion fund, that the insurer will fail would generate hundreds of millions in returns.

The Warburg lifeline for MBIA may be bad news for Ackman.

"We always look for unique opportunities to invest in differentiated franchises with talented management teams," said Warburg Managing Director David Coulter.

The Warburg investment also affords the private equity company the right to nominate two directors to MBIA's board.

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