The history of private sector unions in the US is a positive one. They provide an important balance between the opposing forces of management and labor. I am absolutely in favor of voluntary labor unions that advocate on behalf of their members.
The key reason private sector unions work is that they stand opposite management. This creates a market where labor and management express competing interests. Collective bargaining allows them to establish a mutually-agreed upon price for labor.
Public sector labor unions are a different story.
The feedback loop between public sector unions and elected officials is corrosive. They end up working together against the best interests of the taxpayer. Officials cater to unions for votes; and unions in turn support for those who promise to cater to them. The actual performance of work becomes secondary to the achievement of political ends.
Voluntary, private sector labor unions are a unique expression of freedom. Public sector unions are not. As President, I will support and encourage the former, and strive to diminish the later.