A lot of people are answering that you should not bring politics to the workplace. On one hand this depends, on the other hand, it is not your workplace. You do not have the job and you don't want it.
I think, telling the truth about your reasoning is the best you can do, because:
- You won't lose anything. It is not your workplace (or can you lose anything? if so, you may have to be more cautious). As pointed out by RichardU, Trump is in the US quite an emotional topic which already led to non-rational behavior of people in the past. Even though realistically you won't face any disadvantages, it is still worth to keep the former in mind when talking to people.
- It won't hurt anybody because it's not the company's fault (and if the company generally supports the leader... well even better not to take the job and saying why). So it is nothing "negative" but a matter of facts that you are not willing to accept these political conditions. Do you think it would have been "negative" not to work in a third-Reich factory because of the political conditions?
- It's one of the small things we can do in life that may make a difference. This is actually quite a strong statement as, even though not statistically significant, it has a more significant impact on an emotional level. Declining a job is an action that says "my reason is quite good" and shows some real consequences. Think of the boss hearing of this case, he may talk with others about it, even though it is one case, it is one case (the "I-know-a-guy" effect). But of course, it depends on how important this point is to you.
My advice: decline the job and make the best out of it. I think at least for yourself you will feel quite more comfortable after having made a statement than remaining silent. If you prefer the latter, stating "personal reasons", as mentioned in other answers as well, should be fine.
To the down-voters: please leave a comment and explain why you think it is a bad answer. We all want answers to improve and discussions to happen!