Three personalities he put on the air at Fox during that period stood out for the way they gave voice to a particular kind of American grievance. There was Glenn Beck, whose show debuted the day before the Obama inauguration in 2009. There was also Sarah Palin, who joined as a paid contributor earning $1 million a year in 2010.

And of course there was Donald Trump. He was “relatable rich,” Mr. Ailes told his staff, betting that viewers would see something aspirational in him, when he decided to give Mr. Trump a weekly morning slot in early 2011.

But it was what Mr. Ailes did not see about Mr. Trump — how his popularity was a double-edged sword — that led him to the same flawed assumption that the leaders of the Republican Party would eventually make. What neither they nor Mr. Ailes considered fully as they opened their arms to these insurgent forces was what would happen if encouraging and empowering them meant redefining the limits of acceptable political discourse, dropping the bar ever lower, and then discovering that they were helpless to reel it back in.

That’s how Fox News landed in a once-unthinkable position behind CNN and MSNBC in the ratings in the weeks after Election Day in 2020, losing viewers to outlets like Newsmax and One America News eager to revel in — and profit from — the kind of misinformation that Fox rejected when it told its audience the truth about Mr. Trump’s defeat in Arizona.