Suppose X is the case. When you say “X” your opposite will believe Y, which is wrong. So, even though “X” is the truth, you should not say it.
Your new idea as I understand it: Suppose saying “Z” will let your opposite will believe X. So, even though saying “Z” is, technically, lying, you should say “Z” because the listener will come to have a true believe.
(I’m sorry if I misunderstood you or you think I’m being uncharitable. But even if I misunderstood I think others might misunderstand in a similar way, so I feel justified in responding to the above concept)
First I dislike that approach because it makes things harder for people that could understand, if only people would stop lying to them or prefer to be told the truth along the lines of “study macro economics for two years and you will understand”.
Second, that seems to me to be a form of the-end-justifies-the-means that, even though I think of myself as a consequentialist, I’m not 100% comfortable with. I’m open to the idea that sometimes it’s okay, and even proper, to say something that’s technically untrue, if it results in your audience coming to have a truer world-view. But if this “sometimes” isn’t explained or restricted in any way, that’s just throwing out the idea that you shouldn’t lie.
Some ideas on that:
Make sure you don’t harm your audience because you underestimate them. If you simplify or modify what you say to the point that it can’t be considered true any more because you think your audience is limited in their capacity to understand the correct argument, make sure you don’t make it harder to understand the truth for those that can. That includes the people you underestimated, people that you didn’t intend to address but heard you all the same and people that really won’t understand now, but will later. (Children grow up, people that don’t care enough to follow complex arguments might come to care).
It’s not enough that your audience comes to believe something true. It needs to be justified true believe. Or alternatively, your audience should not only believe X but know it. For a discussion on what is meant with “know” see most of the field of epistemology, I guess. Like, if you tell people that voting for candidate X will give them cancer and the believe you they might come to the correct believe that voting for candidate X is bad for them. But saying that is still unethical.
I guess if you could give people justified true believe, it wouldn’t be lying at all and the whole idea is that you need to lie because some people are incapable of justified true believe on matter X. But then it should at least be “justified in some sense”. Particularly, your argument shouldn’t work just as well if “X” were false.
I understand your post to be about difficult truths related to politics, but you don’t actually give examples (except “what Trump has said is ‘emotionally true’”) and the same idea applies to simplifications of complex material in science etc. I just happened upon an example from a site teaching drawing in perspective (source):
The author way lied to about the possible number of vanishing points in a drawing. But instead of realizing the falsehood he was confused.