According to a report out this morning from the U.S. Labor Department, the number of American workers belonging to labor unions declined again in 2010, continuing a long slide that began in the 1950s.

The number of unionized workers in the private sector fell by 339,000 last year, while the number in the government sector fell by 273,000. The share of private-sector workers belonging to a union dropped to 6.9 percent in 2010, the lowest “union density” in more than a century.

Union leaders are running out of excuses. They can’t simply blame a slowly recovering economy; the number of non-union workers employed last year actually increased by a small amount. And they can’t blame a hostile federal government; President Obama and the Democratic Congress that organized labor helped elect in 2008 tried to give unions just about everything they wanted during the 111th Congress.

The seeds of organized labor’s decline lie within the movement itself. Private sector unions are literally pricing themselves out of America’s increasingly competitive and open markets.

As I pointed out in an article [PDF] for the Cato Journal last year, unions are effective at raising wages and benefits for their members, but not at raising productivity. The result is a “union tax” on certain U.S. companies and sectors, a tax that puts them at a competitive disadvantage against non-unionized firms, resulting in a long-term loss of market share and reduced employment for union members.

Unless America’s private-sector unions change their approach and work more collaboratively with employers in the marketplace, they will continue to commit slow suicide.