Union membership rates have been declining over the past few years with significant drops in the private sector. Contributing factors for the decline include changing composition of jobs and laws that make it challenging to unionize. Although the decline in unionization is heavily apparent in the US, unionization rates in Europe reveal a similar trend. Recent research from economists at Princeton and Columbia University suggest that unionization rates are correlated to income inequality.
See "Union membership in the US keeps on falling, like almost everywhere else", Dan Kopf, Quartz, February 5, 2019