Last week, a lawyer working on Mr. Trump’s behalf, Charles J. Harder of Harder Mirell & Abrams in Beverly Hills, Calif., sent an 11-page cease-and-desist letter to the publisher of Mr. Wolff’s book, “Fire and Fury: Inside the Trump White House.”

Mr. Harder’s letter demanded that the publisher, Henry Holt and Company, withdraw the book from stores and apologize; the publisher responded by moving up the book’s release date and increasing its first print run to one million copies, from 150,000.

Mr. Trump’s remarks on Wednesday about libel law seemed, at times, to refer obliquely to the book, which debuted at No. 1 on the New York Times nonfiction best-seller list, and has provided fodder for dozens of news articles, opinion pieces and cable news segments.

“We want fairness,” the president said. “Can’t say things that are false, knowingly false, and be able to smile as money pours into your bank account. We are going to take a very, very strong look at that, and I think what the American people want to see is fairness.”

As a presidential candidate, Mr. Trump made sport of the reporters who stood in fenced-off areas during his speeches, often whipping up the crowd against them.

He also said on the campaign trail that he would “open up” the country’s libel laws — although he later backed off that pledge in an interview with editors and writers at The Times, joking that he personally might be in trouble if the laws were loosened.

“Somebody said to me on that, they said, ‘You know, it’s a great idea softening up those laws, but you may get sued a lot more,’” Mr. Trump, who propagated false rumors that Barack Obama was born in Africa and that the father of Senator Ted Cruz had aided the assassination of John F. Kennedy, said at the time.