His Fox News colleague Jeanine Pirro also appeared on the rally stage that night. “Do you like the fact that this man is the tip of the spear that goes out there every day and fights for us?” she said, to cheers.

For all his many faults, Mr. Ailes understood the value of maintaining at least the semblance of separation between the network and the political party he was effectively commandeering from his desk in Manhattan. And he believed he had to protect his stable of news correspondents and producers to give Fox News some credibility beyond the core viewers who tuned in for its opinion hosts.

So, for instance, when Mr. Hannity went to Cincinnati to headline a planned Tea Party event in 2010, the boss forced him to cancel, angry that he had even said yes to such a thing.

These days, it seems, Fox News doesn’t have anyone drawing the line. It has been that way since the departure of Mr. Ailes, who was booted from the network in 2016 after multiple women accused him of sexual harassment and who died the next year.

Before he died, Mr. Ailes went to work as an adviser to Mr. Trump. He would not be the last Fox News alumnus to make that move. The network’s former co-president, Bill Shine, followed in Mr. Ailes’s footsteps this year when he became the president’s deputy chief of staff, overseeing communications.

Mr. Shine left Fox News at a time when the output from its pundits increasingly matched Mr. Trump’s initiatives and outbursts. Mr. Hannity has called the investigation by the special counsel, Robert S. Mueller III, part of a “deep state” conspiracy run by a “crime family.” Laura Ingraham has likened the federal detention facilities holding migrant children to “summer camps.” And Tucker Carlson has described the caravan of asylum seekers as “highly dangerous.”