Geithner is already enduring intense criticism from several members of Congress, including many who have demanded that he step down in light of Friday's downgrade of U.S. debt by Standard and Poor's. However, Geithner said over the weekend that he would stay on at President Obama's request.

The May letter from Johanns and other senators cited comments from Geithner, who said last year's healthcare law would strengthen Medicare's financing. But it also said others, including Foster, have indicated that billions of dollars in funds cut from Medicare would be used to fund other entitlement programs, and could not be used to improve Medicare's financial position.

The letter also cited other comments from Obama last April indicating further cuts to Medicare might be needed.

"A joint letter signed by both of you explaining how the above conflicting statements can be reconciled is the only way to clear the air on this subject," they wrote in May. "At a time when our nation is experiencing record-breaking budgetary shortfalls, it is vital that the discussion about one of the largest portions of our federal government is open, honest, and free of ambiguity and contradiction."