The White House claims that the president's senior officials have "absolute immunity" from congressional subpoenas. That stance led more than a dozen current or former Trump officials to refuse subpoenas related to the impeachment process of the president.
House Democrats, however, withdrew their demand for his [Kupperman's] testimony last month, citing the length of time it would take to litigate the case. Without Kupperman's testimony, the House started the formal impeachment process on Tuesday, after weeks of hearings with more than a dozen other witnesses. In court, House lawyers said Democrats will neither hold Kupperman in contempt nor attempt to subpoena him again.
Nevertheless, Kupperman has pushed ahead with his lawsuit, citing fears of being held in contempt of Congress and violating his oath of office.