Partisan animus began to rise in the 1980s, & it has grown dramatically over the past two decades. As partisan affect has intensified, it is also more structured; ingroup favoritism is increasingly associated with outgroup animus

Iyengar, S. and Krupenkin, M. (2018), The Strengthening of Partisan Affect. Political Psychology, 39: 201–218. doi:10.1111/pops.12487

Abstract: Partisanship continues to divide Americans. Using data from the American National Election Studies (ANES), we find that partisans not only feel more negatively about the opposing party, but also that this negativity has become more consistent and has a greater impact on their political participation. We find that while partisan animus began to rise in the 1980s, it has grown dramatically over the past two decades. As partisan affect has intensified, it is also more structured; ingroup favoritism is increasingly associated with outgroup animus. Finally, hostility toward the opposing party has eclipsed positive affect for ones' own party as a motive for political participation.