I think your question involves a false assumption...
Not all Democrats, Not all leftists are the same.
We make a mistake when we make false assumptions.
Some Democrats/leftists really are ideologues who believe what they say.
Some are in it for the money, power and fringe benefits. (Bill Clinton loved the fringe benefits.)
Some are in it because they are lonely and feel a nee for a place to belong.
Some are in it because they depend on the welfare state and its bureaucracy,.
Some are in it because they are crazy, mentally ill.
Some are in it because their granddaddy was in it in the 1960s.
Some are in it and there is no obvious reason why.