Fear is rooted in our physiology, specifically the Amygdala. We all have it, it's there to protect us. If you almost get run over by a bus, or suddenly hear a loud noise behind you, it's the amygdala that drives the flight/freeze/fight response.
Our brain take in stimuli constantly and makes a decision about whether it should scare us, and it all happens at a subconscious level. You can control how you react to the fear, but you can't really control having it or not. So let's not pretend fear is not something that all humans have, or that it's somehow a weakness or character flaw to have it.
I would agree that it's often an irrational thing, and it is sometimes rooted in ignorance, but there's often at least some legitimate concern buried in there somewhere.
For example, the environment is very important to me. I don't think we should pollute to the level we do, and I think we should at least be looking at climate change science and continuing to delve into whether or not we are in fact causing it and what we can do to correct/stabilize it. Because if it is something we can do something about, and it does turn out that some of the more dire predictions are true, what's at stake is whether or not our planet remains hospitable to us. No matter what, even if people are skeptical, we should continue to investigate it if nothing else.
You don't have to agree, but that's how I see things. That's not based on ignorance, it's based on my own conclusions based on all the facts I have seen.
So here's my fear: I've seen several republican presidencies in my lifetime, and I know what a republican presidency looks like for environmental protections (spoiler alert: not good). And Trump's stances sound like they mirror the classic stances in this regard. My fear is that his administration will enact laws that will roll back existing environmental protections and allow pollution to increase, and that they will pull back from the progress we've made globally on climate change.
So is that fear based on ignorance? I don't think so. It's because the thing I care about isn't going to get the attention I feel it deserves, and it's extremely predictable that it will go down this way.
This same principle is true of a lot of issues that people have concerns with. We can't really know what Trump will do, but we do know what previous Republican presidencies have done, and we can see where he has similarities and differences.
Similarly, if Clinton had been elected, right now many supporters would be afraid that border controls would not be enforced and that illegal immigration would impact their livelihoods in some way. I could say those are ignorant fears, but that's not fair either. Just because those things don't directly impact me doesn't mean they aren't real and justifiable fears to the people who have them, and it doesn't mean that they're ignorant because they have cause for concern.