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Donald Trump and Hillary Clinton during the second presidential debate.
But the former Democratic candidate remembers the exact moment she began to take Trump seriously.
"It wasn't until I saw him dominate a debate with a crowded field of talented Republican candidates - not with brilliant ideas or powerful arguments but with ugly attacks that drew gasps - that I realized he might be for real," Clinton wrote in her newly released campaign memoir "What Happened."
It was at the first Republican debate, in August 2015 in Cleveland, that Trump sought to back up the provocative, racially-charged rhetoric on which he launched his candidacy, and where he introduced himself to a national audience that largely viewed his campaign as a sideshow.
On stage with nine other leading GOP candidates, Trump blasted political correctness, famously squabbled with moderator Megyn Kelly, and refused to rule out an independent campaign should he not win the Republican nomination. Many of his comments drew a mixture of raucous applause and loud jeers from the audience.
Two months earlier, Trump announced his bid for the presidency at Trump Tower in New York in a speech that cast Mexican immigrants as rapists and drug dealers.
"He reminded me of one of those old men ranting on about how the country was going to hell in a handbasket unless people started listening to him," Clinton recalled in her book.
"I thought it was important to call him out for his bigotry, which I did early and often," she added.
Clinton then fast-forwarded to a year and a half later, in January, as Trump was getting sworn in as the 45th president of the United States.
"Now here he was, with his hand on the Bible, promising to preserve, protect, and defend the Constitution," Clinton wrote.
"The joke, it turned out, was on us."