The House Committee on Oversight and Reform threatened Thursday to hold a Department of Justice official in contempt of Congress after the agency refused to comply with a subpoena seeking testimony and documents related to the 2020 census citizenship question.

Rep. Elijah Cummings, D-Md., the committee chairman, sent a letter dated Wednesday to Attorney General William Barr instructing him to make John Gore, a principal deputy assistant attorney general, available to answer questions related to Commerce Secretary Wilbur Ross' decision to add a citizenship question to the census.

"The committee expects Mr. Gore to testify in accordance with the Committee's lawful subpoena and the Committee's rules," Cummings wrote in the letter shared with NBC News on Thursday by a committee spokesperson. "If Mr. Gore fails to comply with the subpoena, the Committee will consider him to be in contempt of Congress."

Gore, who was slated to testify Thursday, did not appear. The Justice Department did not immediately respond to a request for comment.

Cummings' move comes weeks before the Supreme Court is set to consider whether the Trump administration's addition of the citizenship question violates the Constitution and the Administrative Procedure Act. Three federal judges have already ruled that Ross' actions were unconstitutional and violated federal law.

Ross, whose agency oversees the Census Bureau, faced tough questioning last month from Democrats during a committee hearing about whether he lied to Congress about his decision to add the question.

Gore emerged as a key figure during the hearing. Ross claimed he decided to add the question in December 2017 after he learned that the Justice Department might want it included, a rationale Democrats rejected.

The committee voted earlier this month, 23-14, to issue a subpoena to compel Gore to testify and the Trump administration to provide additional information about the citizenship question.

Assistant Attorney General Stephen Boyd wrote in a letter earlier this week to the committee that he would not make Gore available unless a department representative accompanied him. However, committee rules prohibit Justice Department lawyers from attending. Boyd, however, claimed the rule "would unconstitutionally infringe upon the prerogatives of the executive branch."