This as well as other parts of your comment seem to imply that you believe the sincerity or intensity with which one holds a particular belief is one of the most important factors in the merits of that belief. I personally do not think this is the case.
You can either assume everyone who disagrees with you is evil or stupid, or you can probably more accurately recognize that someone who disagrees with you, especially if they also live in America, has access to just as much info as you, has access to similar resources, probably isn’t evil: so the conclusion is that they are acting in good faith but with different morals.
Sincerity is a short hand for the fact that most humans are intelligent like you, and most humans in your proximity have access like you, and arent evil, so the result is that they are much more similar than you than you think but with different experiences.
It means you can cut through to the root questions instead of using far worse shorthand’s than sincerity - like “they voted the other way so they much be evil and hellbent on destroying my way of life”.