Ummmmm ...... the definition of "primarying" is subjecting an incumbent to a primary challenge. Kelly Westlund was the D Party candidate in 2014, but she lost to the R, Sean Duffy, ~40% to ~60%. This was in WI's 7th Congressional District. Duffy was the incumbent in that general election (and he may have benefited from Scott Walker's coattails), and (obviously) will be in 2016.
So all mo-mo really means by, "they will be primarying her," is that Kelly Westlund will have competition in the 2016 primary, and the D Party will probably be supporting her competitor. The NERVE!. If Kelly Westlund projects the kind of entitled attitude mo-mo does, she may defeat herself by turning off her would-be constituents.