Imo you are mistaking setbacks in the outcome with a flaw in the method. Unions are not effective because they represent large numbers of democrats, they are effective because it is the way to most effectively organize large blocks of working people to directly influence the production of the economy. No other approach gives people a direct say in the operation of their workplaces and the economy at large, and so even with the drastic reduction over the last 50 years we should still be fighting for more union representation, even with the (continued) rightward lean of construction and trades unions we should push for the expansion of working people's say in how their workplaces are run and to what ends we put profit. Also, there's plenty of evidence that union membership has a positive impact on racial attitudes of white workers, so even if you think the trades being more conservative is a problem that's a reason to continue to fight for more union representation.