That would be a first if the Teamsters endorsed a Pubbie. My guess is the leadership won’t do so outright, but won’t stop their members from doing so (only a guess given the head honchos are in the pockets of Dims and vice versa). That said, IIRC the Teamsters management wanted the pipeline and Obama nixed it (Hillary was wishy-washy on it as I recall).
Neither may endorse but I think that Trump’s trade policies are definitely resonating with them, except that may be offset by union management not wanting the wall and wanting the illegal workers to bring into the fold to get more dues.
I’m thinking why shouldn’t they back him. Trump has worked with unions all his life.