WASHINGTON — A House panel voted on Wednesday to subpoena Kellyanne Conway for her testimony after she failed to show to a hearing at which a special counsel told the committee she should be fired from the White House for her “egregious, repeated, and very public violations” of federal ethics law.

The House Oversight and Reform Committee’s action against Ms. Conway escalates the standoff between the Democratic House and President Trump as the White House stonewalls Democratic oversight inquiries, moving to keep the deliberations of its top officials confidential. The White House blocked Ms. Conway, a counselor to the president, from testifying about allegations of repeated violations of a federal ethics law that prohibits government officials from engaging in political activities at work.

Her failure to show set up yet another clash between the executive branch and Congress that may end with an administration official held in contempt of Congress. Representative Justin Amash of Michigan, the one Republican who has called for President Trump’s impeachment, joined Democrats to vote for the subpoena, 25-16.

In pointed testimony, Henry J. Kerner, the special counsel, whose work is unrelated to the office that was run by Robert S. Mueller III, detailed how the White House counselor’s conduct created an “unprecedented challenge” to his ability to enforce the federal ethics law, known as the Hatch Act.