Well actually, he did a lot of things for the auto industry when he was President.
Like what?
Is it? Or is it a brilliant tactical pollical move to undercut your rivals' historical supporters?
If this were 2015 when voters were still trying to wrap their heads around what a Trump presidency would look like, I'd agree with you. But by now most voters, including union voters, have made up their minds whether they love or hate Trump. The ones who love him would vote for him even if he didn't visit their union and the ones who hate him are not going to change their minds after one visit.