Story highlights Newt Gingrich was a vocal advocate for Trump during his campaign

The former House speaker called the investigation into Trump a "witch hunt"

Washington (CNN) Former House Speaker Newt Gingrich said Friday that it's impossible for President Donald Trump to obstruct justice because of the fact that he's president.

"Technically, the President of the United States cannot obstruct justice," Gingrich said at the National Press Club in an event to promote his new book, "Understanding Trump." "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's got to do is fire him."

However, Gingrich himself has in the past voted to impeach a president over such charges. While speaker of the House, Gingrich voted in 1998 to impeach President Bill Clinton on charges of obstruction of justice and perjury.

Now, with a Republican in power, Gingrich has changed his tune. On Friday he called the investigation into Trump a "witch hunt."

"The left right now is engaged in the Salem witchcraft process of, 'We know somebody's evil, we know somebody's bad. I wonder who we should burn at the stake? Maybe it's you, whoever you are,'" Gingrich said. He encouraged the audience to read Arthur Miller's "The Crucible," saying "that's the mentality of the left right now."

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