So Trump registered as a Democrat. But he did not do this because of ideology. He registered as a Democrats because the government in Queens, NYC was run by Democrats, and he wanted to solicit favors from zoning board people and others who had been appointed by Democrats.
Later, he decided to pass himself off as Republican. Why? He told us why" because Republicans were stupid and easily bribed and otherwise easily bullied." And he was 100% right about that.