Warren E. Buffett, the billionaire investor, said on Tuesday that he had offered to reinsure the municipal bond portfolios of three troubled bond guarantors but had been rebuffed by at least one of them.

Speaking on CNBC, Mr. Buffett said his new bond insurance firm, Berkshire Hathaway Assurance, was willing to reinsure as much as $800 billion in municipal bonds that have been insured by MBIA, the Ambac Financial Group and the Financial Guaranty Insurance Company.

Mr. Buffett’s offer helped drive stocks higher most of the day, though share prices fell back in the final hour. At the close, the Dow Jones industrial average was up 133.40, to 12,373.41, a gain of 1.1 percent, and the Standard & Poor’s 500 stock index rose 0.7 percent. Shares of financial companies, which might benefit from having a strong firm like Berkshire backing municipal bonds, were up 1.4 percent.

But shares of the guarantors fell as investors considered the ramifications of Mr. Buffett’s statements on their business. The offer would do little to alleviate the problems the companies are facing on the guarantees they have made to investors who hold securities backed by mortgages, consumer loans and other assets. In fact, reinsuring municipal bonds with Mr. Buffett could make the guarantors more vulnerable because they would be left with the riskiest insurance contracts.