The biggest problem with the Democratic Party is its belief that America is white or that only white Americans matter. Everyone else is a "special interest" to be indulged only if it doesn't ruffle white people's feathers.
Republicans, by winning a majority of white voters, have greater legitimacy in their eyes, even when the head of the party is someone like Trump.
That Democrats have effectively turned themselves into stock brokers for "socially liberal" wealthy white people doesn't help.