Former NJ Gov. Chris Christie made his debut as ABC News contributor on Good Morning America to warn that the war against the FBI being waged by President Donald Trump and Republicans will be a longterm problem for his party, and the country.

Former FBI Director James Comey is a friend, Christie told ABC News’ chief political correspondent George Stephanopoulos,  but he “made some pretty big mistakes in 2016 for both parties.”

FBI mistakes should be addressed in a way that is “not so political,” Christie cautioned.

Special counsel Robert Mueller, leading the probe into Russian interference in the presidential election is “not someone to be trifled with and not someone who takes lightly the word of anybody he’s looking at,” Christie said.

His advice to Trump: “In an investigation like this, there is nothing you can do to make it shorter. There’s lots you can do to make it longer.”

But, he added, “Only Bob Mueller really knows what he knows, and we won’t know for a while.” Pundits speculating on the status of the probe, Christie said, are “throwing darts at the wall with a blindfold on. Bob Mueller knows what’s going on.”