It seems obvious that having friends across the political spectrum would mitigate partisanship. Fortunately social science backs up the assumption. In this post Neil Gross cites the work of Diana Mutz* and Casey Klofstad to the effect that cross-partisan friendships promote understanding and tolerance…and decrease ideological entrenchment.
But Gross tells us such friendships are an endangered species. Party polarization has become a problem:
According to the political scientist Shanto Iyengar, 27 percent of Republicans and 20 percent of Democrats said in a 2008 survey that they would be “somewhat upset” or “very upset” if their son or daughter married someone in the opposite party. Those figures contrast with 5 percent and 4 percent, respectively, who said they would be “displeased” at the prospect of a cross-party marriage in 1960. [see Iyenger, "Affect not Ideology..." pg. 13]
One solution Gross suggests is to find ways to promote cross-party friendships among college students:
…one might hope that college would be a place where long-lasting relationships with politically diverse others could be forged, offering a bulwark against polarization nationally.
Gross, however, is quite sensitive to the challenges facing such a task. There is ideological sorting in college choice, especially among conservatives who feel at odds with left-leaning academia. And aside from this, it might require "institutional redesign" to get business and English majors in a position to make friends.
*A qualifying caveat: Mutz found that having politically diverse friendships correlated with decreased political participation.