So they'll get richer overall and a part of them will lose their jobs.
> He promises to label China a currency manipulator, but the only consequence of that would be that America would have to engage China in talks on the currency’s value, which would have no practical effect.
The downside is pissing off China, which the US needs to get along. There is no upside -- China won't change its behavior because of a label.
The difficulty will be in keeping a straight face during talks designed to go nowhere.
Whether or not they are genuinely pissed off, they act offended at the bargaining table and make negotiations more difficult than they might have been. They have been known for doing that kind of thing in the past because it plays very well with the people at home.