Mr. Porter didn't dispute any specific elements of "Fear: Trump in the White House", in which veteran Washington Post reporter Bob Woodward writes that Mr. "I've seen this over the years, going back to the Nixon case".
Woodward went on to say, "People who work for him are anxious. that he will sign things or give orders that threaten the national security or the financial security of the country, or of the world".
And Woodward, the victor of two Pulitzer prizes, said, "I'm not".
Trump has dismissed the Woodward book as a "scam" filled with "made up" quotes.
For instance, Woodward writes in Fear, Trump wanted to withdraw from a key trade agreement with South Korea in September 2017. Trump asked, according to Woodward.
Then-economic adviser Gary Cohn reportedly told Trump that he couldn't raise the top tax rate for the simple reason that he was Republican and would "get absolutely destroyed" if he did so. "We're in Crazytown", Kelly said, according to Woodward. "People took actions to protect the country".
As part Trump's efforts to discredit Woodward on Monday, he wrote another tweet in which he quoted Washington Examiner reporter Katelyn Caralle as saying, "You can not take this book too seriously".
Mattis issued a statement, tweeted at least twice by Trump, saying, "The contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence".
"We're doing this in order to prevent World War III", Mattis told the president, according to Woodward's reporting.
And the U.S. ambassador to the United Nations, Nikki Haley, said that she had never heard Mr Trump talking about assassinating the Syrian president.
Pressed on whether Trump can win a credibility battle with the Pulitzer victor who helped expose President Richard Nixon's Watergate-era transgressions, Sanders pointed to "actual, on record" accounts from White House staffers, again pointing to Mattis and Kelly, "not disgruntled former employees".
"I don't know who it is", Woodward says.
In his first of what promises to be an exhaustive round of media interviews, the noted journalist who, along with Carl Bernstein, won the Pulitzer Prize for coverage of Watergate, told David Martin of CBS News, "You look at the operation of this White House and you have to say, Let's hope to God we don't have a crisis". "It's critical: Who is this person and why are they masking themselves in this way?"
Trump has already tweeted his low opinion of the book.