Of course, he didn't originate it. He was an actor not a writer. It was first spoken by Gaylord Parkinson, the chair of the California Republican Party back in 1966 when Reagan ran for governor.
It was a very different time forty five years ago. There actually were moderate and even liberal Republicans. More than that, an actor running for public office was still a novelty, and the other Republicans in the race against him slammed Reagan hard as being an empty suit. They also painted him as being from the Goldwater wing of the party, the group that had been responsible for a truly epic loss against LBJ only two years before.
If you go to the Wikipedia site, you'll see that Reagan broke this rule in his run against Ford in 1976, but in general for many years, the Republicans kept their internecine battles to a minimum, at least in front of the cameras.
Then came 2008 and Sarah Palin. The vast majority of the slams against her came from the opposition, but several conservative pundits hit her on her obvious flaw of experience. After the stinging electoral defeat, John McCain became a non-person, George W. Bush went back to Texas to lick his wounds and the media fell in love with Palin, even after she quit the job of governor to seek fortune commensurate to her fame. For months on news sites of every political persuasion, she was the only conservative voice, and due to a mixture of boredom, envy and self-preservation for their party, Republicans in office, out of office and never holding office began to let their true feelings about the heroine of the trailer trash set come through. Palin, who has never shrugged off a perceived slight in her life, started fighting back, and the 11th Commandment of Saint Ronald Reagan was a dim memory at best.
Enter Donald Trump. It's hard to say if the businessman with the multiple bankruptcies actually is a billionaire or just plays one on TV, but his coy "Will I or won't I?" run for the presidency has catapulted him to the lead in several polls, based in large part to his huge advantage in name recognition.
If Palin is prickly and narcissistic, Trump is just a flat out self-aggrandizing asshole. He will say something nice about someone only as a prelude to saying something nicer about himself. While the polls being taken this far away from a general election are as substantial as cotton candy, they are the closest thing to reality that anyone has to go with, so Trump's non-candidacy candidacy is being taken seriously, most notably now by the pro-oligarchy Club For Growth, who recognize Trump as a traitor to his class for saying a tax increase on the rich would be the best way to shrink the deficit back about ten years ago.
Up until 2004, the Republicans allegedly were a coalition of the pro-Jesus people and the pro-Business people, and both groups saw George W. Bush as one of their own. But the stinging loss of 2008 has shown the cracks in those two camps and emboldened the racists, who have always been there, as Californians who saw the policies of failed governor Pete Wilson can attest. Trump, a man with no scruples and no subtlety, thinks he can bring the racists on board by embracing not only birtherism but several goofy and unsubstantiated Internet rumors about Obama, including the one about Bill Ayers writing Dreams of My Father.
Right now, Trump's success looks like The Fred Thompson Effect, named for the actor and senator who was supposed to be a game changer in 2008 but turned out to be no such thing when he actually ran. The excitement about Trump is really the conservative voters saying en masse, "There's got to be somebody better than these palookas." Should he really run, a few weeks or months in the actual political spotlight and Trump will be outed as just another palooka, and the cry for someone else will continue.
Some commentators are seeing Obama's numbers go down and saying it spells trouble, but even so, the Republicans have to put an actual person up against him, and the field so far looks astoundingly weak, The Donald included.