Using a large, calculated propaganda apparatus, Republicans spent decades building a cult of angry, hateful subservient morons devoid of critical thinking abilities and itching for white nationalist conspiracy theories to validate their anti-intellectual stupidity and victim complex. Republican politicians could get away with fueling that fire because their racist, nationalist agenda was buried beneath a couple layers of politely phrased dog whistles and buzzwords that gave them some level of plausible deniability to claim that they weren't outright racists and bigots.
Then along came Donald Trump, a severely incompetent rich kid business failure/con man with crippling narcissistic personality disorder who decided to dedicate his life to fraudulently marketing himself as successful rich personTM and attention whoring as much as humanly possible to publicly maintain that illusion and profit from it. During the Obama administration Trump correctly calculated that the group most susceptible to his grift was the GOP base so long as he told them what they wanted to hear. So Donald, being a racist, bigoted piece of shit himself, had zero issues spearheading an insultingly stupid racist conspiracy theory about the first black president being born in Africa. This made him a regular fixture on Fox News and allowed him to creep into the collective consciousness of racist moron Republicans.
There's a very believable rumor that Trump decided to run for president to give himself more leverage in a contract negotiation for The Apprentice, but either way, running for President became the perfect attention whoring vessel for Donald to satiate his narcissism and part some rubes from their money like he'd always done. But his strategy was to dispense with the deniability and subtle dog whistling and be as much of a noxious, toxic, openly racist piece of shit as possible, and sink to depths no other Republican politician dared to go. And in doing so, and because of his existing birtherism notoriety, he instantly won the hearts and minds of the racist morons Republicans had spent decades cultivating as the core of their base.
Trump suddenly played the Republicans' own game harder than they were ever willing to, and completely hijacked their base out from under them. Republicans became so subservient to Trump because he became the literal god of their voting base. Then evil people much smarter than Donald latched on to him and used him as a tool to advance their own agendas and elevate him to success by refining his grift. Trump and his cabal successfully exploited Republican stupidity to create his own hate cult with himself at its head, so the GOP had no choice but to worship lord Donald as well, or they risked losing the base of loyal morons they spent so long creating. They completely lost control of the monster they created.
Donald Trump is a worthless, unintelligent narcissistic piece of shit grifter with zero priorities other than satiating his paper thin toddler ego. It's all been an attention whoring con. He's never cared about anyone or anything but himself, much less the United States. His presidency was the perfect storm of political stupidity led by the GOP's decades long brainwashing campaign to turn impoverished white people into racist morons. Trump was both a calculation and an accident. A monster created by American fascism and the people who have allowed it to grow out of control.