A conservative group filed a federal lawsuit Wednesday seeking to have the federal health care law declared unconstitutional, arguing that the government postponed its so-called employer mandate without the necessary congressional approval. The lawsuit came the same day that a congressional panel grilled Kathleen Sebelius, the secretary of Health and Human Services, over problems that overwhelmed the government website intended to help people shop for health insurance. The Association of American Physicians and Surgeons is seeking to have the health care law overturned. The group is seizing on a provision that was supposed to go into effect next year but ultimately was postponed for 12 months.