As Ms. Barrett recounted in an interview with The Washington Post this year, she asked him about the presidency for no other reason than that he began complaining about the state of the world, lamenting that other nations were taking advantage of the United States. (He said he had no interest in running.)

Oprah Winfrey followed with the same question in 1988, after Mr. Trump published his smash hit book “The Art of the Deal” and became an outspoken critic of the trade deficit. When she asked Mr. Trump if he would run, he told her, “Probably not, but I wouldn’t rule it out.” It was daytime television, and it all seemed like good fun.

But then, that same year, he was suddenly talking about it at the Republican convention on CNN, with Larry King, who was, in his way, an open door in the wall that had separated entertainment and news. A decade later, during Bill Clinton’s impeachment trial, it was Rowland Evans and Robert Novak on CNN raising the P-word (president) with him as he defended Mr. Clinton.

All the while, Mr. Trump was making regular visits to the studio of the radio talk show host Howard Stern, where he shared views of women such as, “A person who is flat-chested is very hard to be a 10.” (Mr. Trump recently said such commentary was being offered “for the purpose of entertainment,” apparently as he promoted the beauty pageants he owned with CBS at the time.)

So it was not surprising that such a deep well of incredulity rose up when Mr. Trump emerged as a candidate for the Reform Party’s presidential nomination in 1999. “Is it Campaign 2000 or ‘Entertainment Tonight’?” Howard Kurtz, then the host of “Reliable Sources” on CNN, asked in reference to the media attention to Mr. Trump. Still, Mr. Trump made it all the way to the political proving ground of Tim Russert’s “Meet the Press,” before his bid fell apart a few months later.

Yet as we know now, nothing did more to set up Mr. Trump for 2016 presidential politics than his own TV show, “The Apprentice,” which became a hit during its first season, in 2004. Its huge ratings success made Mr. Trump an even more coveted guest on entertainment shows, like “Access Hollywood” and “Late Show With David Letterman,” and mainstream news programs, which were ever more desperate for ratings and, therefore, ever more willing to embrace celebrities.

Mr. Trump always made it worth their while. He had a willingness to talk about whatever an interviewer threw at him, which is how a one-on-one session with Wolf Blitzer in 2007 came to be dominated by Mr. Trump’s views of the Iraq War (“a disaster” that called for an immediate withdrawal) and his critique of President George W. Bush’s leadership.