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Chris Christie warned U.S. President Donald Trump that attacking the Mueller investigation would be like "poking the bear."

"I told him back in February of '17 that in my experience there was no way to make any of these investigations shorter, but there was a sure way to make it longer," said Christie, a long-time friend and adviser to Trump.

"And that would be to be talking about it and tweeting about it," he said.

Christie, a former U.S. attorney who also served as governor of New Jersey, has known Trump for 17 years. He was one of the first prominent Republicans to back Trump's presidential bid, and also led Trump's transition team before being fired just days after the 2016 election result.

Donald Trump, left, talks with Chris Christie, right, during at a campaign rally in Hickory, North Carolina, during the presidential campaign, on March 14, 2016. (Sean Rayford/Getty Images)

He has written about his experience inside the Trump campaign in his new book Let Me Finish: Trump, the Kushners, Bannon, New Jersey, and the Power of In-Your-Face Politics.

In his only Canadian interview, he told The Current's Anna Maria Tremonti that Trump's attacks on special counsel Robert Mueller's investigation are borne out of an emotional response, and not "something that's an intellectual process."

The president has repeatedly referred to the investigation — into alleged collusion between the Kremlin and Trump's presidential campaign — as a "witch hunt."

The single greatest Witch Hunt in American history continues. There was no collusion, everybody including the Dems knows there was no collusion, & yet on and on it goes. Russia & the world is laughing at the stupidity they are witnessing. Republicans should finally take control! —@realDonaldTrump

"I think that the president is enormously frustrated by the fact that he, in his heart, believes he did absolutely nothing wrong, viz-a-viz Russia, and that this has now dragged on for nearly two years, and has really placed a dampener on his presidency," Christie said.

"I think there are times when he is able to control the frustration and other times he just simply is not able, or willing," he said.

Last week, acting attorney general Matthew Whitaker said the investigation is "close to being completed."

While Christie doesn't think Trump will be impeached, he said, "we have to wait to see what the report says."

"I think there's a lot of Democrats in the House who would love to impeach him, but I don't know whether they're going to have the grounds to do it with a straight face."

Trump thought Democrats 'would give in' over shutdown

Trump will deliver his second state of the union address Tuesday night.

Christie said it's a chance for the president to lay out his pitch for re-election in 2020, but also an opportunity to reassure the American people that the recent government shutdown won't be repeated.

He just said to me: 'We've decided to make a change,' and I said 'OK, what change are we making?' And he said: 'You.' - Chris Christie

"I do think that it would make some sense for him tonight to let the American people know that he realizes that this shutdown caused some pain, and for that he's very unhappy and won't let it happen again."

He told Tremonti that the shutdown lasted a record-breaking 35 days because Trump "thought that the Democrats would just give in."

"I think that's someone who's not experienced at legislative politics," he said.

A demonstrator holds a sign, signifying hundreds of thousands of federal employees who won’t be receiving their paychecks as a result of the partial government shutdown, during a rally in Washington on Jan. 10. (Carlos Barria/Reuters)

Kushner put 'an axe to my head with Donald Trump'

Christie threw his support behind Trump in Feb. 2016 after his own bid for the Republican presidential nomination failed, in part, due to a scandal involving traffic on the George Washington Bridge.

"Let me be clear, Donald Trump was not my first choice to be president — I was," he told Tremonti.

But faced with a Hillary Clinton presidency, he said he "preferred, and do prefer, having Donald Trump in the White House, with the policies he's pursuing."

Trump picked Christie to lead the transition process, but the appointment was short-lived. Three days after the election win, Christie met with Trump's chief strategist Steve Bannon.

"It was a very abrupt meeting," Christie recalled.

"He just said to me: 'We've decided to make a change,' and I said 'OK, what change are we making?'

"And he said: 'You.'"

Christie is flanked by Trump, right, and his son-in-law and adviser Jared Kushner, left, in a meeting at the White House on March 29, 2017. Christie says Kushner had him fired as head of the president's transition team in 2016. (Jonathan Ernst/Reuters)

Christie says the decision to fire him had come from the president's son-in-law, Jared Kushner.

He believes it was payback for his role in sending Kushner's father, Charles Kushner, to jail in 2005. He had pleaded guilty to 18 counts of tax evasion, witness tampering and making illegal campaign donations.

Bannon told Christie that Jared Kushner had been "working for some time … putting an axe to my head with Donald Trump."

"I understand a son being upset about this father going to prison," he said. "What I don't understand is the unwillingness to acknowledge that your father plead guilty."

"I think he hasn't acted in the best interest of the president, and that bothers me," he said.

Christie says he has stuck by the Trump presidency because he "wants to make it better," pointing out that he also worked with Barack Obama's administration, despite their differences.

"When the president picks up the phone and calls you and asks you to help ... you try to help if you're a patriot, and that's what I am."

Click 'listen' near the top of this page to hear the full conversation.

Written by Padraig Moran. Produced by Howard Goldenthal.