Additionally, how would you even prove "well I didn't hire that person because they belong to the wrong political party" short of someone actually saying it and documenting it against themselves? Racial discrimination laws can be exceptionally difficult to enforce, as I understand it.
In an ideal world, what my employer thinks shouldn't affect my political actions. As usual, real and ideal are far apart.
My union doesn't do collective bargaining negotiations.. I'm a tech worker...
It covers me legally should employers take too much of an interest in my personal life, etc...
They also run a news paper about engineering topics..
Also, not trying to overstate my "struggle" or anything -- the reason I'm active politically is because of the impact on people I care about, not the impact on me. I'm fully aware I'm pretty damn privileged to be able to write what I wrote above.
I've been thinking about this a lot lately in calling and sending emails to my government representatives, attending marches and protests. By definition, these are done in public; there's no point otherwise! But there's nothing preventing my employer from deciding they don't like what I'm putting out there and doing .. something about it.
I don't see anything to be done about this, certainly not in the short term, but it's a very different sort of thing than supposing there's something on social media I would be "embarrassed" about my employer seeing.