Any debate about the future of unions in the U.S. is really a discussion about public sector unions. The private sector's unionized workforce is simply too small to matter outside of a few industries. .
The chart is from Harvard University Labor and Worklife program at the Law School. I also think the relative small size of private sector unions is one reason the hope that unions will be a force for combatting poverty is misplaced. Unions are finding it hard enough to hold on to their place in the middle class.