Fortunately, those union members are starting to realize that they were conned by Trump. By March of this year, their support for Trump had dropped to 47% -- a drop of 15 points. That's still too high. Democrats need to make it clear in this coming election that they (not Trump or his Republican buddies) are the party that helps working men and women.
They can do this by pointing out the anti-union actions Republicans have taken, the broken promises of Trump to workers, and the Trump/GOP tax reform that gave huge tax cuts to the rich and corporations (but next to nothing for workers).
The chart below shows the percentage of workers in a union in each state. The states in red are the ones that voted for Trump.
These charts are from polls done by Reuters / Ipsos.