The answer is Rush Limbaugh, basically. It's a little more than that, but essentially these people are brainwashed by talk-radio and Fox News, according to this documentary: The Brainwashing of My Dad (Prime) by Jen Senko. It happened to her father, the brainwashing, and he transformed over years from a gentle and great parent to a dittohead. It was grueling, and apparently it is suuuper common, losing your aging parents to Rush and Fox. The psychology behind it is fascinating, and evidently (because some of the ex-perpetrators of this propaganda machine have come clean) the brainwashing is completely deliberate. The late Roger Ailes, chairman and CEO of Fox News up until 2016, led the channel like a dictator, and was apparently totally cognizant of his own agenda and how to achieve it (sidenote: unlike Trump, who I strongly believe to be an accidental success not knowing why his style works, but just behaves like that naturally).
Ex-Fox anchor explains how to start deprogramming your Fox-brainwashed friends and relatives (Tobin Smith)
FEAR & UNbalanced: Confessions of a 14-Year Fox News Hitman (Tobin Smith)
Tobin Smith is a former Fox News commentator for 14 years.
According to the documentary, this brainwashing of people functions like a cult, hits elderly people more than anyone else, probably due to normal (mild) cognitive decline. But as for all cult members, there is a way out, and in the case of Senko's dad (spoiler alert!), it was cutting off the radio, television, and emails that targeted him. As a result, he became a reasonable person and father yet again. But many aging parents suffer from the same affliction, which shuts them off to reason and compassion, and notably, they vote. Ultimately, it is a case of the rich and compassionless doing what they do best, which is using their money to influence public opinion to control policy to their own benefit. The more people know about this, the better we can fight back. So watch the documentary for starters.