Donald J. Trump is ending 2017 by declaring war on federal law enforcement.

What a charming way for the president of the United States to sign off on an old year and welcome the new one with forbidding declarations.

He’s gone after the FBI. He is calling it a dysfunctional agency. He has labeled its investigation into whether the Trump campaign colluded with Russians seeking to influence the 2016 election a “witch hunt.” He fired the FBI director, James Comey, this past spring.

Trump cannot stop yapping about how much he detests the investigation, how much he distrusts special counsel Robert Mueller — whom the Justice Department hired to take over the probe.

The president’s continual disparagement of federal law enforcement agencies is troubling at many levels. I’ll just cite a couple.

One is that the FBI has long been held in high regard by Trump’s fellow Republicans. But the party has become the Trump Party. Longtime Republicans have grown infatuated with the man rather than the party’s ideology.

Indeed, the president lacks an ideology. He doesn’t adhere to core principles. His seemingly sole interest is in boosting himself, his brand.

The other level brings me back to a point I want to make yet again. It is that if Trump is as clean as pure-driven snow on the “Russia thing,” he should welcome the special counsel’s probe, not condemn it.

He should allow Mueller’s probe to run its course. He should let Mueller reach a conclusion. If it finds nothing at the end of its journey, then Trump can crow all he wants.

His continual yammering and yapping about Mueller, the FBI and his foes, however, suggests to me that the special counsel may have something to keep pursuing.

And that is what is giving Donald J. Trump fits.