WASHINGTON — White House officials directed Kellyanne Conway on Monday not to comply with a congressional subpoena compelling her to answer to accusations of multiple violations of a federal ethics law, invoking the “longstanding principle of immunity for senior advisers to the president.”

The House Oversight and Reform Committee authorized a subpoena for Ms. Conway last month after Henry J. Kerner, a special counsel for a nonpartisan government watchdog agency, told the committee that Ms. Conway should be fired from the White House for her “egregious, repeated and very public violations” of a federal ethics law called the Hatch Act, which prohibits federal employees from engaging in political activities at work.

But President Trump has vowed to defy all subpoenas, and on Monday, when Ms. Conway was due to testify, Pat Cipollone, the White House counsel, told Representative Elijah E. Cummings, Democrat of Maryland and the chairman of the oversight panel, in a letter that she would ignore the summons. Invoking the claim of absolute immunity, Mr. Cipollone wrote that “in accordance with longstanding, bipartisan precedent, Ms. Conway cannot be compelled to testify before Congress with respect to matters related to her service as a senior adviser to the president.”

The move is the latest turn in a growing confrontation between the House and the executive branch, as they battle over just how far Congress’s oversight power extends and whether a president’s senior aides are “absolutely immune” from subpoenas. The White House has blocked testimony from a number of the president’s top aides, current and former, invoking that claim.