Trump should not sit for an interview with Mueller: Chris Christie Only Mueller knows what he knows, Christie said.

 -- The president should not sit down with special counsel Robert Mueller - a lawyer, former FBI director and "not someone to be trifled with” - warned former New Jersey governor Chris Christie Tuesday on "Good Morning America" in his first interview as an ABC News contributor.

After his own presidential run ended, Christie became a key player in President Donald Trump’s campaign and the first to head his transition team. He’s also a former U.S. attorney.

“[Mueller’s] not someone who takes lightly the words of anybody who he's looking at,” Christie said. “So what I've said all along is that in an investigation like this … There’s nothing you can do to make it shorter. There’s lots of things you can do to make it longer."

There are other ways to cooperate with the investigation besides an interview with a special counsel, he said.

“I don't think the president of the United States, unless there are credible allegations which I don’t believe there are, should be sitting across from a special counsel. The presidency is different. I don't think they should do that,” he said.

He also rejected the contention that the president needs to answer questions about possible obstruction of justice.

“That’s a very high standard to meet … I don’t think we’ve met that high standard yet as to President Trump, but we’ve got to continue to watch it,” he said. “Because the one thing I can tell you for sure, and one of the things I loved about being a prosecutor, was only I knew what I knew. Only Bob Mueller really knows what he knows and we won't know it for awhile."

Christie also has ties to FBI director Christopher Wray, who was his personal lawyer during the George Washington Bridge scandal known as “Bridgegate,” and someone he worked under during his time in the Justice Department.

He believes Wray's decision to replace Andrew McCabe as FBI deputy director was made independent of the White House.

McCabe had long been expected to retire in March but has now stepped down - taking leave until then. He has been the target of angry tweets from the president.

But the decision on McCabe's future came down to Wray, Christie said.

“One thing I know about Director Wray -- he will make those decisions himself. No one else will make those for him,” Christie said.

“Remember, whether you agree with [former FBI director] Jim Comey or you don't agree with him, Chris Wray has the right to have his own team in place at the FBI, take the FBI in the direction he thinks it needs to go,” Christie said.

Also making news at the bureau is a classified Republican memo alleging FBI wrongdoing in the Russia investigation, which is now at the White House. President Trump can decide whether to allow it to become public and the sensitive intelligence information it’s said to contain.

Christie urged caution.

“You've got to be careful to go down this road because once you do, you know, the worm can turn on you too,” he warned. “So I think this is something the president should really carefully consider. I'm sure he probably has. My guess is that he's going to release it,” Christie said.

The memo is part of a campaign by Republicans to call attention to alleged misconduct by the FBI in the Russia investigation.

“You see the president and so many House Republicans waging a war against the FBI. This seems to be a flip of the script,” ABC News’ chief anchor George Stephanopoulos said.

“It does and I also think it's a long-term problem for our party if we continue this. Not only for our party but for the country,” Christie responded.

As Trump takes the stage for his first State of the Union address Tuesday night, he needs to turn the page and get back to being “the president who is speaking to [American] concerns,” Christie advised.

And the Russia investigation? Don’t mention it, Christie said.

“I don't think there's anything he can really say about it that makes any sense. I think if he gets in and gets down that rabbit hole, he tends to get angry about it and that's not the person you want to see tonight,” he said.

Christie will join ABC News’ live coverage for the State of the Union tonight, starting at 9 p.m. from the Hawk ‘n’ Dove restaurant and bar on Capitol Hill.