■ Two weeks later, Mr. Trump declared he would not run, citing his “passion” for business and a new contract with NBC for “Celebrity Apprentice.”

He was gone, but the anger of many G.O.P. voters remained. Rick Santorum, not Mr. Trump, wound up being the insurgent who gave the party establishment fits. He came a lot closer to beating Mr. Romney than a lot of people remember (and it’s not hard to imagine that Mr. Trump could have done even better).

In 2011, Mr. Trump had a chance to give his media strategy a test run. The New York Times article on his farewell from the race suggested that the most noteworthy element of his flirtation as a candidate was “a media culture that increasingly seems to give the spotlight to the loudest, most outrageous voices.” Stuart Spencer, a former political strategist for Ronald Reagan, was quoted as saying, “The media made him, the media kept him, the media kept promoting him.”

Mr. Trump also demonstrated his willingness and ability to mine racial and ethnic resentment. In 2011, Mr. Trump said, “China is raping us.” Four years later, he said Mexico was sending rapists to the United States.

In the run-up to the Trump candidacy of 2016, Gabriel Sherman reported in New York magazine that an employee of Mr. Trump, Sam Nunberg — who was later fired for racially charged Facebook posts under his name — measured the base’s pulse.

“I listened to thousands of hours of talk radio, and he was getting reports from me,” Nunberg recalled. What those reports said was that the G.O.P. base was frothing over a handful of issues including immigration, Obamacare and Common Core. While Jeb Bush talked about crossing the border as an “act of love,” Trump was thinking about how high to build his wall.

But maybe more than anything, Mr. Trump showed in 2011 how he would deploy conspiracy theories. Being a birther was merely a start. He associated with conspiracy purveyors like Alex Jones, a syndicated radio host. Among many examples in the last year, The Times wrote in March, Mr. Trump “reposted information on Twitter from the website Infowars, hosted by Mr. Jones,” to support his unsubstantiated claim that thousands of Muslims in New Jersey cheered the Sept. 11 attacks.