The chairman of a House civil service subcommittee called yesterday for the firing of an Education Department regional director and a political appointee in the Treasury Department, after the panel investigated how a private citizen's post card to the government provoked a stinging response calling America "a Christian nation."

Rep. Patricia Schroeder (D-Colo.) asked for the removal of Thomas G. Tancredo, head of the Education Department's Denver office, and Christopher Sundseth, a self-styled Christian activist assigned to the Inter-American Development Bank. She made the request in letters to Education Secretary William J. Bennett and Treasury Secretary James A. Baker III.

Schroeder's investigation began when Gerald B. Leib of Mountain View, Calif., wrote to Tancredo criticizing him for distributing a speech that referred to America as a Christian nation. The reply to Leib's post card came not from Tancredo, but from Sundseth, who called Leib an "amazing, pathetic creature" who espoused "small-minded tripe."

Schroeder accused Sundseth of conduct "inappropriate and unbecoming a federal employe," both for using his office to obtain the post card and for replying to Leib with a "derogatory" letter.

She said Tancredo should be fired for "extensive mismanagement" of his Denver office and for obstructing the House panel's investigation. During the probe, Tancredo said he did not keep copies of letters sent to the office and had no recollection of Leib's post card.