The problem with advocating Socialism is that the term has been viciously abused by many malign people who would have found a better home in the far right that claiming to be on the left. Robert Mugabe claims to be both Socialist and Marxist, and is quite clearly neither.

That said, I think Marxism has failed to progress much in the 20th century. The theories of Das Kapital have been accepted unchanged and Marxist-Leninism has been practised without adaption since Lenin's death. The cult of personality and failure to adapt to new situations and enviroment stalls the potential of the Left.

In answer to the original question, Socialism would fail in the US because market values are too strongly engrained for people to accept concepts of collective benefit and egalitarianism. After decades of the Soviets being the (actually worrying) boogyman, people in the US are hardly likley to adopt the theories their arch enemy claimed to follow.

I get the feeling that quite alot of people don't really know what they support when they declare themselves socialist, I have a theory that they just think of the ideal outcome and a warm fuzzy feeling of community without considering processes and practicalities.