A liberal lobbying group has asked two members of Congress to investigate how a Christian activist at the Treasury Department obtained a post card mailed to another federal agency.

Anthony T. Podesta, president of People for the American Way, a 150,000-member organization, called for the probe in letters to Sen. Robert T. Stafford (R-Vt.) and Rep. Ted Weiss (D-N.Y.).

The Washington Post reported Wednesday that Christopher C. Sundseth, a political appointee assigned to the Inter-American Development Bank, had obtained a post card sent to Tom Tancredo, the Education Department's representative in Denver.

The card, from Gerald B. Leib of Mountain View, Calif., criticized Tancredo for distributing a speech that called America a "Christian nation."

Sundseth wrote to Leib, saying "we are indeed, like it or not, a 'Christian nation' " and calling him a "truly amazing, pathetic creature."

Sundseth, 31, an acquaintance of Tancredo, said he believed that he obtained the card from one of four friends who regularly use the Freedom of Information Act to obtain correspondence with the government on religious issues. He said he writes many such letters and sees nothing wrong with expressing his beliefs on his own time.

Podesta, accusing Tancredo of "grossly improper conduct," said he plans to examine such practices at the Education Department. He said:

"The evidence is mounting of a direct pipeline from Mr. Tancredo's desk to radical religious zealots who interpret their self-styled biblical mandate as a license to act as a 'religious KGB' . . . . "