I agree with your boss about Trump being terrible -- but that doesn't mean your boss should be able to impose HIS political beliefs on YOU.
I think you may just have to suck it up and endure this, but your boss is abusing his economic position of power over you in order to push his own political agenda.
I don't advise arguing with your boss -- unless you're very, very sure that you won't get fired over this. In the past, from a very leftwing perspective, I've sometimes argued with bosses, and it didn't do my career any good.
Under our capitalist economic system, unfortunately, the "democracy" that we more or less enjoy outside of the workplace almost disappears when we have to earn our livings at some kind of job.
As Marx wrote a long time ago, when your boss or the owner of your workplace controls the "means of production," and you don't, and you also need to earn money to live, you almost become a "wage slave" to the boss or the business owner. Whether Marx identified the solution to this problem or not -- many people think he didn't -- it really is a problem.
Even when we go to work for somebody else, there should be something like "economic democracy" to protect us from the arbitrary power of other people, especially our bosses. The fact that they can control our labor for 8 hours a day (or whatever) shouldn't mean they also get to control our minds.
-- democratic socialist