Eugene Debs and Upton Sinclair were significant leftists.
Debs was a founding member of the Social Democracy of America (1897), the Social Democratic Party of America (1898) and the Socialist Party of America.
Upton Sinclair had a lower profile during his run for California governor in 1934. The Socialist party in California and nationwide refused to allow its members to be active in any other party including the Democratic Party and expelled him, along with socialists who supported his California campaign. That policy destroyed the Socialist party in California. Sinclair had been a member of the Socialist Party from 1902 to 1934. He always considered himself a socialist in spirit and hid his Leftist motivations for his writings—as they all do, even in these times.