Washington(CNN) The House committees leading the impeachment inquiry are hoping to hear from Robert Blair, an assistant to President Donald Trump and senior adviser to acting chief of staff Mick Mulvaney, though that looks unlikely considering Blair's attorney said he has refused to testify in the probe.
The House committees investigating Trump had scheduled Blair's deposition for Monday.
Blair -- who was on the line during Trump's July call with Ukraine's President -- was requested to testify about White House policy toward Ukraine.
"Mr. Blair is caught between the assertions of legal duty by two coequal branches of government, a conflict which he cannot resolve," Blair's attorney Whit Ellerman told CNN on Saturday.
Blair has not yet received a subpoena, but Ellerman said Blair will still refuse to testify if he is subpoenaed.
Blair, who was associate director for national security programs in the Office of Management and Budget, followed Mulvaney in January to the White House when Mulvaney became acting chief of staff. Mulvaney made Blair an assistant to the President. Blair serves as Mulvaney's senior adviser for national security issues.
Blair's hiring allowed Mulvaney to have a hand in national security issues without having to go through former White House national security adviser John Bolton. After Bolton was fired, one administration official said that Blair could be a favorite to replace Bolton because of his support from Mulvaney.
Blair was one of just a small group of officials on the line during Trump's controversial phone call with Ukrainian President Volodymyr Zelensky. Mulvaney was not.
During the July 25 call, Trump asked Zelensky to investigate former Vice President Joe Biden -- despite there being no evidence of wrongdoing by either Joe Biden or his son, Hunter, in Ukraine. The phone call was part of a whistleblower's complaint that alleged Trump sought "to solicit interference" from Ukraine in the upcoming 2020 election, and that the White House took steps to cover it up. Trump has denied doing anything improper.
Before joining the Trump administration, Blair worked for the past 14 years as a staffer for several committees in the House of Representatives. His last position was staff director on the House Subcommittee on Defense Appropriations.
He previously worked as a regional adviser for Africa at the US State Department of State from 2001-2003.
According to his LinkedIn profile, he received a bachelor's degree from Cornell University and two master's degrees from Tufts University. He served as a Peace Corps volunteer in Africa during the mid-1990s.