The Democrats have been fighting for the poor for 50 years and the poor have increased.
The number of "poor" depends on how it's defined.
Fifty years ago, poor people didn't have free phones, air conditioning, dishwashers, microwaves, cable color TV, computers, automobiles, $100 gym shoes, etc., etc., etc..
If we use a reasonable definition of "poor", more like that used by the rest of the world, they've nearly disappeared.
To maintain their base, democrats need to convince more and more people that they're poor.