As a teacher you should understand this phenomenon. Leftists are very much like children who rebel against authority and test limits. But like children, they actually do listen and care what the grown-ups think. I have a variety of leftist to semi-leftist friends and acquaintances. They spout off from time to time and I tell them how I support my president and his policies. If they were true believers and dedicated to their cause that would be the end of it - they would cease the conversation and remain in their own little world. But 9 times out of 10 they keep coming back and wanting to engage again and again and again. I think they are like kids who secretly know their parents, or their teachers or other authority figures are right but are just testing limits or repeating what they heard on the school ground. So it's important to state the truth even if you think they are not listening because 9 times out of 10 they are listening.
But like children, they actually do listen and care what the grown-ups think. I think you have nailed it here. My liberal/moderate friends always seek out my opinions, especially around election time. They want to know "what the hate-talkers are saying" about various issues. Even long time liberals realize their thought process does not always jive with reality. And they are amazed I know so many things they don't hear on the MSM.
It's wrong to stay silent when we can help influence the thoughts of those around us. We won't be able to convert everyone, but there are alway those in the middle who can be persuaded with facts and common sense.
When confronting die hards just keep smiling. Libs hate that. They are the most humorless people on earth.