Great Divide

Artificial line of separation that determines whether a discussion flows left, right or goes nowhere. Inhospitable and difficult to cross, the great divide has turned middle ground into a largely unoccupied region.

Also reflecting differences between social and political cultures that become accentuated when the parties are unable to mix easily with the opposite side.

Source: From the North American continental divide which follows the Rocky Mountains and generally defines the parting of the waters in the U.S. See Across the Great Divide

Related term: Polarization, common ground, across the aisle