Once upon a time, back when America had a strong middle class, it also had a strong union movement.

These two facts were connected. Unions negotiated good wages and benefits for their workers, gains that often ended up being matched even by nonunion employers. They also provided an important counterbalance to the political influence of corporations and the economic elite.

Today, however, the American union movement is a shadow of its former self, except among government workers. In 1973, almost a quarter of private-sector employees were union members, but last year the figure was down to a mere 7.4 percent.

Yet unions still matter politically. And right now they’re at the heart of a nasty political scuffle among Democrats. Before I get to that, however, let’s talk about what happened to American labor over the last 35 years.

It’s often assumed that the U.S. labor movement died a natural death, that it was made obsolete by globalization and technological change. But what really happened is that beginning in the 1970s, corporate America, which had previously had a largely cooperative relationship with unions, in effect declared war on organized labor.