Offering a political opinion at work nowadays is like tossing a Molotov cocktail into an open fire. With issues from gun rights to gay rights polarized past the point of civil discussion, it's important to avoid comments on politics whenever possible.
There are two ways to go about this. The first is passive. When someone at work asks you for your opinion, the safest course is not to respond verbally at all, even with body language that may somehow suggest an attitude. Most people will accept this as their cue to cap any more questions about candidate X or issue Z.
But while most will let it go right then, others might be paradoxically provoked by your silence - believingit implies a rejection of their party, candidate, or issue. That's where a second and more active approach might be useful.
If the other person persists in badgering you, an effective response is: "Not much good happens when people discuss their political views in the workplace, so I prefer to keep my opinions private. One of the great things about our freedom of speech is the freedom not to speak - and when it comes to politics, I choose not to." Then change the topic or disengage.
Like religion, sex, and other touchy topics, the most politically correct way of handling politics is to keep them out of the workplace.