Its never pleasant to learn what you thought was true, was indeed false; that what you thought your hero stood for, was actually a sham. We've all been there, although usually at much younger ages, and with fictitious characters ... not elected officials.
In situations like that, the civil thing to do is admit you were wrong, and perhaps be a little less vehement next time lest ignorance be displayed again.
And for the rest of us, I told you so just doesn't quite cover it here, does it. For those that realize the error of their voting record, and there are quite a few and growing, we should just nod and move along cheerfully. There will however always be those rabid in their willful ignorance, who will continue to resort to Ad Hominum arguments as the facts persistently undermine their opinions. For them there's not much that can be done.
I have tried to do the same with voters from both ends of the progressive party (democrat and republican) and found it has opened doors to civilly discuss liberty.