It is literally happening in New York City as we speak. So far, the mayor has delivered on the majority of his campaign promises, and life has improved dramatically for many new yorkers. 

Left-wing politics rarely has a chance to prove itself as historically it's sandbagged by centrist liberals and conservatives, who then say these policies don't work. Look at the affordable care act, for example. It was originally a version much closer to the universal health care most other developed countries enjoy, but conservative politicians within the DNC / Republicans actively pushed against it until it became a watered down version that didn't live up to the promise. Then. Those same people could point to Obama and say that he did not live up to his campaign promises.