Vandalism and Politics
Here in Arizona, I've seen quite a few Bernie Sanders bumper stickers and a small but roughly equal number of Hillary Clinton and Donald Trump stickers.
Bumper stickers and campaign buttons aren't as popular as they used to be. They may be regarded as somewhat downscale or simply as a pain to remove. Way back in 1964 when passions were high and favorite son Barry Goldwater was running against President Lyndon Johnson, there were plenty of bumper stickers for both candidates and yet I never heard of anyone's car being vandalized because of a sticker.
That tolerance has been gone for some time. It is not unusual to hear people mention that they'd never put a political sticker on their car. The primary reason cited is fear of vandalism. The specific fear is that the car will be "keyed." That is a cruel and extreme way of harming someone who simply expresses a different opinion.
The "bumper sticker fear" may be a small reflection of what has happened to political discourse.