The United States doesn't really have a left-wing party. There's the Green Party and the Socialist Party, but neither of them is relevant in any meaningful way. I suggest that you vote with the Greens or Socialists, if you're truly interested in left-wing politics, even if they are irrelevant. It may not accomplish much, but you'll be able to sleep better at night. If you're more of a centrist or right winger, then I suggest the Libertarian Party, which are at least supportive of freedom, even if they are free market fundamentalists. I can respect their stance on freedom, at the very least... which is more than I can do for most political parties.
There's also the Social Justice Party, but I don't know much about them. The Greens piss me off every once in a while, with their anti-technology, neo-luddite rhetoric. Social Justice seems like a decent alternative, if you're into progressive, left-wing politics and don't want to go full-on socialist.