Megan Romer likes to spend her free time sewing costumes for school plays, picking blackberries in her backyard in rural Cayuga County, N.Y., and watching her partner play the fiddle in folk bands.  In her day job, she is seeking to overthrow capitalism. Romer, 42 years old, is the national co-chairwoman of the Democratic Socialists of America, the far-left organization whose candidates have swept to victory in several recent high-profile races by preying on Democratic incumbents. The next showdown comes next Tuesday, when Congressional primaries in Missouri and Michigan again pit Democratic socialists against incumbent Democrats. “This is what we’ve…