Bond insurer MBIA (NYSE: MBI) said hedge fund founder William Ackman's proposal for a restructuring of U.S. bond insurers is no more credible or viable than his flawed open-source model, The Wall Street Journal reported [subscription required].

"Like Mr. Ackman's open-source model, his statements in the media and the barrage of letters he has sent to regulators and the rating agencies -- which contain half truths, innuendo and faulty analysis -- this proposal is simply a continuation of Mr. Ackman's campaign to profit from his short positions and credit default swaps in the bond insurance industry," MBIA said. MBIA added that it is continuing to work with New York State Superintendent of Insurance Eric Dinallo and his advisers to evaluate options for maintaining the highest rating for its policyholders.

Furthermore, MBIA, the nation's largest bond insurer, said it agrees with a spokesman for the New York Insurance Department who said Ackman's proposal would split the company and likely lead to a substantial downgrade for the structured side.

Amid calls for disclosure of more information on bidding for auction-rate bonds after dealers stopped buying the securities, two economists told BloggingStocks Friday that the problem of a lack of investor demand speaks directly to the need to re-capitalize bond insurers MBIA and Ambac.

"The problem is not merely a lack of demand for bonds. The problem is that institutional investors are shunning these investments because they are concerned about a lack of available insurance for this debt and related credit market uncertainty, which underscores the need to address the liquidity concerns of MBIA and Ambac," economist David H. Wang said Friday.

MBIA, Ambac: two linchpins

The bond insurers, Wang said, are two linchpins of the bond market [municipal, corporate], and, by extension, of the financial markets.

Shares of MBIA (NYSE: MBI) and Ambac (NYSE: ABK) have lost more than 70% of their value in the past six months, as investors have fled them amid concern that the two do not have sufficient capital to fund insurance policies for mortgage-backed and collateralized debt obligations held by banks and institutions. MBIA and Ambac executives have rejected the accusations, arguing that they have sufficient capital to fund claims and can modify/improve their business models, long-term, aided by re-capitalization. MBIA fell 80 cents to $11.82 and Ambac fell 45 cents to $10.08 in Friday afternoon trading.

Barclays analysts say banks that obtained $72 billion in funding to replenish capital depleted by subprime-related losses may need another $143 billion in capital infusions if credit rating agencies downgrade bond insurers several levels, Bloomberg News reported Friday.

Barclays analyst Paul Fenner-Leitao Banks wrote in a research report published Friday that banks will need at least $22 billion if bonds covered by insurers MBIA (NYSE: MBI) and Ambac (NYSE: ABK) are cut one level from the current AAA and six times that if they are cut four levels, Bloomberg said. The capital amount is based on Barclays' estimates that the banks hold as much as 75% of the $820 billion of the structured securities guaranteed by bond insurers.

Meanwhile, the markets awaited word on New York Insurance Superintendent Eric Dinallo's meeting with banks on a bail-out package for bond insurers. Shares of some key bond insurers fell after Dinallo issued a statement that the negotiations were complicated and would take time, leading some in the market to doubt the New York agency's ability to marshal private resources for the initiative.

MBIA fell 79 cents to $13.61, Ambac gained 15 cents to $11.48, PMI Group (NYSE: PMI) rose 17 cents to $8.97, and MGIC Investment (NYSE: MTG) declined 6 cents to $16.68.