Michelle Bachmann was once a Democrat. You could conclude several things from that. One is that anything is possible, another is that people can change over time, and a 3rd is that fundamentally, especially when it comes to the free enterprise system, Democrats and Republicans aren't all that different.
Are there socialists in this country that vote democrat because they see them as the lesser of two evils? Probably. Are there Socialists that run for office pretending to be Democrats? Maybe, though I doubt they get very far. Belief in free enterprise (tempered by regulation) is part of the Democratic platform. If someone like Fidel Castro were to join the Republican party, would that make him a Republican?
The current trend of labeling Democrats or their proposals as "socialist" is the same tactic as calling someone a "liberal" was 20 years ago. The difference is that the term "liberal" doesn't have same stigma "socialist" does and was losing its effectiveness. What is especially ironic is when you hear someone refer to Obamacare as "socialist". A socialist would consider Obamacare an abomination (or Obamination if you prefer). It's not that the are opposed to universal health care, just the way Obamacare attempts to achieve it. As far as that goes they share common ground with Republicans or Libertarians.