There was a time when the United States had a robust and growing middle class that was the envy of the world. Sadly, that is disappearing. The Republicans have never liked labor unions, and when they seized power again, they instituted anti-union policies along with a propaganda campaign denigrating unionism.
As you can see from the top chart (from the Huffington Post), that war against unions has been disastrous for the middle class. As union membership declined, so did the wages of middle class -- and the middle class declined also.
The only saving grace at this point is that a majority of Americans (second chart) see the declining union representation as a bad thing for workers (51% bad to 35% good). But that 51% needs to grow. That means that Democrats and unions need to do a better job of telling their story to Americans. The public needs to understand that labor unions are the only way they can effectively bargain with employers (especially large employers).
The bottom chart is from a Pew Research Center survey -- done between April 25th and May 1st of a national sample of 1,503 adults, and has a 2.9 point margin of error.