In this Oct 17, 2019 photo, acting White House chief of staff Mick Mulvaney speaks in the White House briefing room in Washington. (EVAN VUCCI / AP)

WASHINGTON — White House acting Chief of Staff Mick Mulvaney on Monday withdrew his request to join a lawsuit seeking a court ruling on whether witnesses must testify in the US House of Representatives impeachment probe into President Donald Trump, saying he would bring his own case, according to a court document.

White House acting Chief of Staff Mick Mulvaney has emerged as a central figure in the impeachment inquiry, partly because of his statement at an Oct 17 news conference that the White House had withheld security assistance for Ukraine

Mulvaney had earlier sought to participate in a lawsuit filed by Charles Kupperman, a former deputy to ousted national security adviser John Bolton, seeking a court ruling on whether he should comply with a congressional subpoena or honor the Trump administration's order not to testify.

Mulvaney withdrew the request following a conference call closed to the public held by the judge assigned to Kupperman's lawsuit, US District Judge Richard Leon in Washington.

According to a transcript of the hearing, Leon said he was "inclined" to deny Mulvaneys request to intervene and encouraged him to instead file his own lawsuit.

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Last week, House Democrats withdrew their subpoena to Kupperman, saying they did not want to delay the impeachment investigation, and asked a judge to dismiss the litigation as moot.

Mulvaney "can't intervene into a moot case," Todd Tatelman, a lawyer for the House, said during Monday's hearing.

House investigators issued a subpoena to Mulvaney last week, demanding he testify about his knowledge of Trump's decision to withhold military aid to Ukraine.

Congressional Democrats are trying to determine whether Trump withheld the assistance to pressure Ukraine's government to investigate the son of former Democratic Vice President Joe Biden, one of Trump's main rivals as he seeks re-election next year.

Trump denies any wrongdoing.

Mulvaney has emerged as a central figure in the impeachment inquiry, partly because of his statement at an Oct 17 news conference that the White House had withheld security assistance for Ukraine.

"I have news for everybody: Get over it. There is going to be political influence in foreign policy," Mulvaney said at the time, although he later contradicted himself.

Before becoming acting chief of staff, Mulvaney ran the White House Office of Management and Budget, which made the decision to block nearly us$400 million in security assistance for Ukraine last summer.

The White House has instructed current and former Trump administration officials not to cooperate with the impeachment investigation, arguing in court filings that the US Constitution allows presidential aides to defy demands by Congress for testimony.

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Mulvaney's lawsuit could lead to a ruling on that argument by Leon, a conservative appointed by former Republican President George W Bush, who is already assigned to Kupperman's case.

US District Judge Ketanji Brown Jackson, an appointee of former Democratic President Barack Obama, is weighing similar questions in a lawsuit the House filed to enforce a subpoena issued to former White House lawyer Don McGahn.