Prevailing Wage Laws.
Most (all?) states have a prevailing wage law and the federal government has one caled the DavisBacon Act.
Prevailing wage laws are just another sweetheart deal between government and unions to promote unions without specifically saying that is their purpose. These days the government isn't so subtle.
Prevailing wage laws don't specifically require union labor on public works projects but they do require all work be paid at the "prevailing wage" rate. The prevailing wage is usually the wage paid by the unionized companies. So the non-union companies are not-qualified to bid unless they increase their wage rates to union levels.
The DavisBacon Act of 1931 is a United States federal law which established the requirement for paying prevailing wages on public works projects. All federal government construction contracts, and most contracts for federally assisted construction over $2,000, must include provisions for paying workers on-site no less than the locally prevailing wages and benefits paid on similar projects.
These are like $14-$16/hour jobs. No more than that.
Works for me...the prevailing wage is zero when you're unemployed, so you don't have to do the work for free; we'll just go out for COMPETITIVE BIDS, Union or Non-Union, and see who is the best bidder without interference.