My theory on partisanship today is that people treat elections and political parties like sports contests and favorite sports teams. Once they commit to the left or right, they support that "team" regardless of the circumstances. Partisans won't listen to any type of fact or reason that suggests that their team may be in the wrong. Your team can be terrible, but you will cheer for that team no matter what.

It's no longer about good government, but only about winning. It explains the irrational bashing of Bill Clinton, George W. Bush, Barack Obama, and now Donald Trump. I don't think it was as bad during the administrations of Ronald Reagan or George H. W. Bush.

How Congress became so partisan, in 4 charts - The Washington Post