Former House Speaker Newt Gingrich (R-Ga.), a staunch supporter of President Trump, said Friday that a "president cannot obstruct justice."

Gingrich said that as president, Trump was well within his authority to fire the head of the FBI.

“The president of the United States is the chief executive officer of the United States. If he wants to fire the FBI director, all he has to do is fire him,” said Gingrich in comments at the National Press Club, where he was promoting a new book.

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His comments sparked pushback on Twitter, with many noting that Gingrich had voted to impeach former President Bill Clinton William (Bill) Jefferson ClintonAnxious Democrats amp up pressure for vote on COVID-19 aid Barr's Russia investigator has put some focus on Clinton Foundation: report Epstein podcast host says he affiliated with elites from 'both sides of the aisle' MORE, including an article that charged him with obstruction of justice.

Obstruction of justice was the third article of impeachment against Bill Clinton in the House. And Newt Gingrich was Speaker of the House. https://t.co/ldDjphH05P — Bradd Jaffy (@BraddJaffy) June 16, 2017

In 1998, Gingrich voted for an impeachment article that charged Bill Clinton with obstruction of justice https://t.co/VtOiutkudb — Jamie Dupree (@jamiedupree) June 16, 2017

Gingrich's comments came after Trump in a tweet earlier this morning said that he was being investigated for firing former FBI Director James Comey.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," the president tweeted.

It was unclear what information the president was using to make the claim. Trump was also likely referring to Deputy Attorney General Rod Rosenstein, who penned a memo last month questioning Comey's handling of the probe into Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE's use of a private email server while secretary of State.

The White House initially used that memo to justify Trump's decision to fire Comey.

Rosenstein also tapped former FBI Director Robert Mueller to be special counsel and oversee the probe into Russian meddling in the 2016 election, including any ties between the Trump campaign and Moscow.

Reports earlier this week said that probe was being expanded to cover whether Trump obstructed justice by firing Comey.

Comey testified before Congress last week that the president had asked him to drop a probe into former national security adviser Michael Flynn, before his firing, and called the Russia probe a "cloud" over his administration.

Gingrich slammed Mueller, accusing him of packing his team with Trump opponents.

”Mueller is surrounding himself with people that are conducting a witch hunt," said Gingrich.

The former Speaker on Friday was touting his new book, "Understanding Trump."