Mr. Horowitz, citing requests from members of Congress and the public, spent 17 months examining the F.B.I.’s handling of the Clinton email case. His conclusion: There was “no evidence” that the decision not to seek charges against Mrs. Clinton was “affected by bias or other improper conclusions,” the opposite of what Mr. Trump had been asserting for months.

But during that investigation Mr. Horowitz uncovered hundreds of texts between an F.B.I. agent, Peter Strzok, and an F.B.I. lawyer, Lisa Page, that suggested animus toward Mr. Trump and also revealed that the two had in the past engaged in an extramarital affair — information eagerly disseminated by the Justice Department and Trump allies.

Since then, Mr. Trump has tweeted about Ms. Page over 40 times, caricaturing her and Mr. Strzok as “love birds” conspiring to bring down the president, with Mr. Trump often using the most vulgar terms to whip his supporters into a partisan frenzy. At a rally in October, Mr. Trump simulated an orgasm while saying: “I love you, Peter! I love you, too, Lisa! Lisa, I love you. Lisa, Lisa! Oh God, I love you, Lisa.”

Citing that incident as the last straw, this week Ms. Page sued the Department of Justice for unlawfully releasing the texts, which she said had “radically altered” her day-to-day life.

The existence of an investigation provides the president and his allies with unlimited opportunities to speculate about the outcome, while the inspector general is bound by confidentiality restrictions until the report is released. Senator Lindsey Graham, Republican of South Carolina, confidently predicted the inspector general’s report would demonstrate a “system off the rails” before he read it.

This may help explain why Mr. Trump, in his efforts to pressure Ukraine’s government to open investigations of Joe Biden and Hunter Biden, didn’t really care whether the Ukrainians actually conducted such an investigation — only that one be announced. That would have given him and his allies the opportunity to speculate about what the investigation was finding and to tar the Bidens without any risk that an investigation would exonerate them.

It doesn’t matter if the report itself turns out to be something of an anticlimax. To his credit, Mr. Horowitz didn’t abandon the objective evidence in an effort to please his overseers. He certainly didn’t reach the answers about Russia or the Clinton email investigation for which President Trump and his allies so fervently hoped.