After doing some research, it appears that Frelinghusen's district is heavily republican. It seems unlikely that he will be defeated. Due to redistricting in the 80s, the district has gone puke since then. Here is a link that discusses this ..
https://en.wikipedia.org/wiki/New_Jersey%27s_11th_congressional_district
(from the above link)
New Jersey's 11th Congressional District is represented by Republican Rodney Frelinghuysen. The district is centered in Morris County, with suburban settlements and a high per capita income; it is dominated by Republicans. The territory is located in the area of the Watchung and Ramapo Mountains.
Prior to a redistricting in the early 1980s, the 11th was centered in Essex County. The congressional seat was held by Democrats for over 40 years. The redistricting, conducted under a Republican-dominated legislature, shifted the focus of the district to Morris County, whose population was dominated by Republicans. In 1984, Republican Dean Gallo defeated 22-year incumbent Democrat Joseph Minish. Since then, the district has been one of the most reliably Republican districts in the Northeast. The Democrats have not made a serious bid for the seat since Minish's defeat.