Ex-White House aide to Peter Navarro gave a final speech on Monday before reporting for his prison sentence, making him the first former White House official to be imprisoned for a contempt of Congress conviction. 

The one-time adviser to former president Donald J. Trump was sentenced to four months in prison for his refusal to comply with a subpoena to appear before the House Select Committee during the investigation into the January 6, 2021 attack on the US Capitol. 

Navarro spoke at a gas station for 30 minutes and railed against an "unprecedented assault on the constitutional separation of powers."

Navarro claimed that the legal tactics used against him would be against Trump. "I am pissed - that's what I am feeling right now." Navarro concluded: "God bless you all, see you on the other side."

Navarro's prison stint comes as Trump himself wades through multiple criminal trials and accusations, though Trump has repeatedly denied any wrongdoing. 

"It's historic, and will be to future White House aides who get subpoenaed by Congress," Stanley Brand, a former House general counsel who now represents Navarro as one of his defense lawyers, said on Monday.

There has been a long standing dialectic between the executive and legislative branches of the U.S. government over protections surrounding the presidency and how Congress can enforce subpoenas.  Both sides have been incentivized to negoiate deals rather than test those powers of separation in court.

This raises important questions on executive privilege and immunity. The Justice Department prosecuted the former White House adviser for skipping out on a congressional subpoena after Congress held him in criminal contempt. He was then referred to the Department of Justice. 

Prosecutors posit that Navarro's non-compliance with lawmakers put him outside the normal bounds of discussion other former officials have had with lawmakers over their participation in congressional probes. 

Navarro unsuccessfully petitioned the Supreme Court to intervene on his behalf.

"The prosecution of a senior presidential advisor asserting executive privilege conflicts with the constitutional independence required by the doctrine of separation of powers," his lawyers wrote to the high court.

"Not once before Dr. Navarro's prosecution has the Department of Justice concluded a senior presidential advisor may be prosecuted for contempt of congress following an assertion of executive privilege." 

His request was rejected by Chief Justice John Roberts on Monday.