While membership in unions is still really low, unions are enjoying a 70% approval rate - the highest that it's been in 70 years. Labor unions have shown that they still have some muscle lately, from the epic wins by Shawn Fain and the UAW to the successful organizing of Starbucks workers. In other words, it's not a group to blow off if you happen to be running for office, say something like President of the United States.
Team Harris Walz knows the score and are making a concerted effort on Labor Day.