I suspect I could come up with more counterexamples if I were a better student of history, but, at least in recent memory, there has generally been a reluctance for certain writers, artists, directors, etc. to explicitly endorse candidates through their work. That's not to say that these people did not take partisan positions, but that they often maintained something of a firewall between what they said on their own and what they said"on the job ."
For example, Randall Munroe of XKCD was an Obama supporter, but he chose to keep politics out of the cartoon he posted the day before the 2008 election
You can put it down to Paul Krugman's line about Trump as clarifying shock or mine about Trump as stressor, but either way, the stark contrast in this election has caused a lot of people to abandon (at least temporarily) a number of long-held conventions.
Here's today's cartoon:
Schlock Mercenary: March 26, 2017
5 hours ago