These might be the most problematic findings in the Pew study. It helps explain the inability of Congress to work together. Tough to do so if you think the opposition is a threat to the nation.
Why the increase since 1994?
Why the - slightly - greater animosity of Republicans towards Democrats?
There is a tactical advantage for political parties to foster antipathy however. It helps get the vote out, and it helps persuade people to donate money.
So while we might conclude that these attitudes are problematic for the long term health of the republic, they can benefit parties in the short term. So this is not completely irrational.