If anything, the PSL is a front for Russia, given that Russia has featured them on Russian state media. But I don't see them as being a threat to the Democrats. A quick look at their Wikipedia page shows that the most percentage votes they ever got in an election was 40%, and that was just 57 votes in a college town.

It's a pity the US doesn't have a parliamentary system. It would make so much more sense to have something like Greens, Socialists, and Democrats forming a coalition.