Thomas S. Condit, former president of D.C. National Bank, will become the new president of the National Consumer Cooperative Bank. The announcement is expected today.

He takes over when the three-year-old institution's fortunes are at an ebb. At an oversight hearing yesterday by the House Banking Committee, Chairman Fernand St Germain (D-R.I.) spoke of "total and unbelievable disappointment as far as the performance of the bank, its board of directors and officers are concerned." The bank must take "swift corrective steps if it has any chance for success," he warned.

Condit, 41, will succeed its first president, Carol S. Greenwald, who resigned under fire last fall.

Condit said he expects to play a major role in formulating policy. He developed expertise in start-up loans for small businesses during his five years at D.C. National.

The Coop Bank was founded to finance nonprofit health, food and housing cooperatives. Only about 40 percent of the more than $200 million put up by the government has been lent. Last year, examiners found that 54 percent of its loans were credit risks and 25 percent were not earning interest. Further deterioration has been found since then, according to Eldon Stoehr, deputy governor and chief examiner of the Farm Credit Administration, who testified yesterday.

St Germain recited a litany of the bank's administrative problems, ranging from a growing alienation from its constituents, the cooperatives, to its board's seeming inability to make hard decisions, to excessive legal fees--more than $1.4 million to outside law firms in 15 months. "The board and management must realize that the way to the hearts and minds of the cooperative movement may not necessarily be through the D.C. Bar Association," he added.