A top national security aide and key ally to acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE will not testify on Monday before House Democrats in their impeachment inquiry into President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE, Politico reports.

An attorney for Robert Blair said his client would not appear on Capitol Hill on Monday “pursuant to direction from the White House, which is based on advice from the Department of Justice,” according to Politico.

The news outlet notes that when asked if Blair would appear to testify if subpoenaed by Democratic lawmakers, attorney Whit Ellerman said his client still would not show, adding that “direction from the White House and advice from [the Department of Justice] cover subpoena.”

Blair, whom Mulvaney brought onto the White House staff less than a year ago, reportedly took part in the July 25 call between Trump and Ukrainian President Volodymyr Zelensky that spurred a whistleblower complaint and is at the center of the impeachment probe.

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Blair has specialized in national security policy and worked on Capitol Hill and served as senior adviser to Mulvaney, Politico noted.

He is one of a handful of White House aides that Democrats have asked to testify this week as they ramp up their impeachment probe.

Officials have also called on John Eisenberg, deputy counsel to the president for national security affairs; Michael Ellis, senior associate counsel to the president; and Brian McCormack, associate director for natural resources, energy and science at the Office of Management and Budget.

It is unclear which of these officials will testify as the White House has insisted the administration will refuse to cooperate with the impeachment investigation. The White House did not immediately respond to a request for comment from The Hill.