When you hear that Trump was a Democrat until the 90's, consider this:

He was building in New York City, particularly Manhattan. Anything you wanted to grease the gears had to more or less go through politicians and their agencies and their underlings. And they were all Democrats. You could donate to candidates, but the real money had to be democratic.

His father, Fred Trump, knew this for decades. He was a Republican, but donated generously to Democrats in NYC and New York State. Price of doing business.

Trump's mentor, Roy Cohn, knew all the politicians and knew where the bodies were buried. He was a Democrat.

Trump followed their lead.

Once Ronald Reagan got elected, and later, when Newt Gingrich took control of the House, Trump followed their lead and converted. Republicans and their supporters were the wave of the future.

Trump has no affinity and affiliation. It's only who will let him make more money and gain more power.