WASHINGTON — The White House is directing Kellyanne Conway, the president’s counselor, to reject a request to testify before a House committee about her repeated violations of a federal ethics law that bars government officials from engaging in political activities at work, further escalating a standoff between Congress and the Trump administration over oversight requests.

In a letter sent on Monday night to Representative Elijah E. Cummings, the chairman of the House Oversight and Reform Committee, the White House counsel, Pat A. Cipollone, cited “longstanding precedent” in declining the invitation for Ms. Conway to appear before the committee.

“The precedent for members of the White House staff to decline invitations to testify before congressional committees has been consistently adhered to by administrations of both political parties,” Mr. Cipollone wrote, citing letters from former counsels to Presidents Barack Obama, Jimmy Carter and Bill Clinton.

The Oversight Committee is now preparing to subpoena Ms. Conway and will move to hold her in contempt if she does not comply, Mr. Cummings said. “We cannot have people disobeying the law. The president is not above the law, and nor is Ms. Conway above the law,” Mr. Cummings said. “There are many, many federal employees who are obedient with regard to the Hatch Act.”