While there are all kinds of radical opinions, in USA there are two quite clearly established camps of mainstream opinion, and it does seem reasonable that if roughly half of the country (or your state) vote for one candidate and roughly half for the other, then it should imply that the opinions of both of these camps are roughly equally valid from the perspective of the society.
One gets a bit more votes and gets to decide policy, then another gets a bit more votes and gets to decide different policy, but ultimately both groups are part of the society and the workforce, and they should be able to coexist and cooperate while working in a single company. Whatever your stance is on the "left" and "right" topics, people who hold and express the exactly opposite stance, an opinion that you consider invalid and who likely consider your opinion on that political topic invalid in return, those people may be your neighbours and your colleagues and they have the same right to be there as you do, what's sauce for goose is sauce for gander.
And the main criterion for the eligibility of a discussion IMHO is not the relevance of the political topic to workplace (IMHO the target of employer's political dollars are just as valid topic as employer's religious mission, i.e. not at all, and specific discrimination of specific employees in this company would be on-topic but general discrimination issues in the country are exactly just as valid as general immigration policy or general tax policy) but the impact the discussion has on inter-employee relations and conflict. A political topic where strongly opposing co-workers can express their opinions, disagree, shrug, and go on working without the disagreement affecting their cooperation is obviously okay to discuss, but if the discussion of some topic results in employees conflicting for those non-directly-work-related political reasons, or in some employees not expressing their opinion because conflict would be imminent, then that topic is harming work and it would be reasonable to require to put it aside. Because letting it be (or, worse, the employer taking on one of the sides) would be de facto creating a hostile work environment for one or the other groups (whichever happens to be the less dominant at that particular workplace, it definitely cuts both ways), and that's simply not acceptable.