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.

Read More