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 has denied. He served as a key witness in former special counsel Robert Mueller's investigation.
Throughout Mueller's report, Porter is mentioned as being in the president's orbit during key events the panel is investigating.
One such episode described says Trump directed Porter to instruct former White House counsel Don McGhan to create a false document denying that the president had asked McGhan to file the special counsel.
"Porter thought the matter should be handled by the White House communications office, but the President said he wanted McGahn to write a letter to the file ‘for our records’ and wanted something beyond a press statement to demonstrate that the reporting was inaccurate," the Mueller report reads.
House Judiciary Chairman Jerry Nadler, D-N.Y., said in a statement released with the subpoena Monday that some instances in the Mueller report involving Porter deserve another look.
"Any other American would have been prosecuted based on the evidence Special Counsel Mueller uncovered in his report," Nadler said. "Rob Porter was prominently featured in the Special Counsel’s description of President Trump’s efforts to obstruct justice by directing then-White House Counsel Don McGahn to fire the Special Counsel, and then ordering him to lie about it."
The subpoena for Porter comes as Democrats on the Judiciary Committee continue to ratchet up their investigation, which earlier this month Nadler said could end in a vote for articles of impeachment.
Nadler said that Porter's testimony will help the "committee determine whether to recommend articles of impeachment against the President ."
"No one is above the law," his statement read.
It's likely that the White House will attempt to block to the execution of the subpoena as it has done with several other subpoenas issued by the Judiciary committee as part of its investigation.
If the White House does attempt to prevent Porter from turning over documents or providing testimony, it will likely claim that Porter's direct communications with the president in his former official role have "absolute immunity" from probing by Congress.
The White House has used a similar argument to block testimony from former White House counsel Don McGhan and to limit the scope of former White House Communications Director Hope Hicks' testimony before the panel in June.
House Democrats have 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 subpoena issued by the committee Monday requests that Porter appear before the committee for testimony on Sept. 17. The committee has already issued subpoenas to Trump's former campaign manager Corey Lewandowski and Trump's former White House deputy chief of staff for policy, Rick Dearborn, to appear for public testimony on the same day.
"These witnesses were all involved in President Trump's extensive efforts to obstruct the Special Counsel's investigation," the committee said in a statement.