Donald J. Trump has leveled an extraordinarily serious allegation against the FBI: that the law enforcement agency spied on his presidential campaign for “political purposes.”

An investigation into that charge has commenced. The Department of Justice’s inspector general is taking the lead.

I am heartened to some degree that the IG is conducting this probe. Attorney General Jeff Sessions recused himself from anything related to the Russia matter, given his own bias as a campaign operative and the role he played in helping formulate the future president’s foreign policy.

The decision to bring in the IG fell to Deputy AG Rod Rosenstein, who appointed special counsel Robert Mueller to lead the probe into the Russia matter.

This battle between the president and the FBI has been unprecedented at many levels already. That the president of the United States would condemn the FBI in such harsh terms, let alone doing the same thing to the Justice Department, is unheard of. Some observers have suggested the president’s strategy to discredit the FBI, DOJ and Mueller may be paying dividends for him in the eyes of the public.

I, as one American voter, find Trump’s strategy to be offensive in the extreme. That’s just me, though. You already know how I feel about Trump and his unfitness for the job to which he was elected.

He’s called Mueller’s probe the “worst witch hunt” in U.S. history, apparently ignoring the fact that in the 17th century, women were actually killed because some colonists thought they were, um, witches.

With all the leaks that have permeated this investigation, it’s fascinating in the extreme that Mueller’s team of legal eagles has been hermetically sealed against such leakage. He has remained silent, preferring to go about the task to which he was assigned: to find the truth about Trump’s election-year relationship — if any existed — with Russian goons who meddled in our election.

I want the inspector general to conclude his own probe in fairly short order. My hope is that he he can root out all the facts and make a reasoned, dispassionate finding on what Trump has proclaimed so hysterically.

However, the slope is mighty slippery. Watch your step, Mr. Inspector General.