In letters sent to the committee on Monday, obtained exclusively by ABC News, White House counsel Pat Cipollone said Porter and Dearborn are immune from compelled congressional testimony.
"As you know ... in accordance with long-standing, bipartisan precedent, senior advisers to the President such as Mr. Dearborn and Mr. Porter may not be compelled to testify before Congress with respect to matters related to their service as senior advisers to the President," the letter from the White House to House Judiciary Chairman Jerry Nadler , D-N.Y., said.
"Accordingly, in keeping with settled precedent and to protect the Office of President for the future, the President has directed Mr. Dearborn and Mr. Porter not to appear at the hearing scheduled for Sept. 17, 2019," the letter said.
The White House also used the "absolute immunity" argument to block testimony for former White House counsel Donald McGahn and to limit the scope of former White House Communications Director Hope Hicks' testimony before the panel and behind closed doors in June.
House Democrats argued in a recent lawsuit filed to enforce McGhan's subpoena that the "absolute immunity" argument has "no grounding in the Constitution, any statutes, or case law" and stating that the president cited "no legal authority" to instruct a private citizen, like McGhan or Porter, to defy a congressional subpoena.
The White House also directed Trump's former campaign manager Corey Lewandowski not to speak about conversations he had with Trump as president and with senior advisers and only about his time working for Trump on the campaign.
"... Mr. Lewandowski’s conversations with the President and with senior advisers to the President are protected from disclosure by long-settled principles protecting Executive Branch confidentiality interests and, as a result, the White House has directed Mr. Lewandowski not to provide information about such communications beyond the information provided in the portions of the [Mueller] Report that have already been disclosed to the Committee."
In response, Nadler said in a statement, "This is a shocking and dangerous assertion of executive privilege and absolute immunity. The President would have us believe that he can willfully engage in criminal activity and prevent witnesses from testifying before Congress --even if they did not actually work for him or his administration. If he were to prevail in this cover-up while the Judiciary Committee is considering whether to recommend articles of impeachment, he would upend the separation of powers as envisioned by our founders.
"No one is above the law," the statement continued. "The House Judiciary Committee will continue our investigation of the President’s crimes, corruption and cover-up and get to the truth for the American people."
Lewandowski, who served as Trump's first campaign manager until he was fired in June of 2016, has remained close to the president and the West Wing, serving as an outside adviser to Trump since his election but never served in Trump’s administration.
Porter served as White House staff secretary for Trump from January 2017 until he resigned amid allegations of domestic abuse in February 2018 -- allegations he denied. He served as a key witness in Mueller's investigation.
Both Porter and Dearborn were on the list of a dozen subpoenas the committee authorized in June, requesting documents and testimony from twelve current and former administration officials and associates of Trump related to their obstruction investigation.
Democrats see public testimony from them and other prominent Trump figures as key elements of their investigation to determine whether to take up impeachment against the president.