It's now official: In 2009 the number of unionized workers who work for the government surpassed those in the private economy for the first time. This milestone explains a lot about modern American politics, in particular the paradox that union clout with Democrats has increased even as fewer workers belong to unions overall.

The Bureau of Labor Statistics reported recently that 51.4% of America's 15.4 million union members, or about 7.91 million workers, were employed by the government in 2009. As recently as 1980, there...