DENVER -- Driving her city bus through downtown Denver, Angela Williams would seem to be one of those "invisible" people Hillary Clinton and fellow Democrats appeal to. She's a Hispanic union member who earns $39,000 a year.

Jim Kelley, whose office Ms. Williams drives by, looks like one of those plutocrats whom Democrats are talking about taxing more. He buys companies for the $7 billion private-equity firm Vestar Capital Partners, with headquarters on New York's Park Avenue.

Think...