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.

Don’t take my word for it; read Business Week, which published an article in 2002 titled “How Wal-Mart Keeps Unions at Bay.” The article explained that “over the past two decades, Corporate America has perfected its ability to fend off labor groups.” It then described the tactics — some legal, some illegal, all involving a healthy dose of intimidation — that Wal-Mart and other giant firms use to block organizing drives.

These hardball tactics have been enabled by a political environment that has been deeply hostile to organized labor, both because politicians favored employers’ interests and because conservatives sought to weaken the Democratic Party. “We’re going to crush labor as a political entity,” Grover Norquist, the anti-tax activist, once declared.

But the times may be changing. A newly energized progressive movement seems to be on the ascendant, and unions are a key part of that movement. Most notably, the Service Employees International Union has played a key role in pushing for health care reform. And unions will be an important force in the Democrats’ favor in next year’s election.

Or maybe not — which brings us to the latest from Iowa.

Whoever receives the Democratic presidential nomination will receive labor’s support in the general election. Meanwhile, however, unions are supporting favored candidates. Hillary Clinton — who for a time seemed the clear front-runner — has received the most union support. John Edwards, whose populist message resonates with labor, has also received considerable labor support.

But Barack Obama, though he has a solid pro-labor voting record, has not — in part, perhaps, because his message of “a new kind of politics” that will transcend bitter partisanship doesn’t make much sense to union leaders who know, from the experience of confronting corporations and their political allies head on, that partisanship isn’t going away anytime soon.

O.K., that’s politics. But now Mr. Obama has lashed out at Mr. Edwards because two 527s — independent groups that are allowed to support candidates, but are legally forbidden from coordinating directly with their campaigns — are running ads on his rival’s behalf. They are, Mr. Obama says, representative of the kind of “special interests” that “have too much influence in Washington.”

The thing, though, is that both of these 527s represent union groups — in the case of the larger group, local branches of the S.E.I.U. who consider Mr. Edwards the strongest candidate on health reform. So Mr. Obama’s attack raises a couple of questions.

First, does it make sense, in the current political and economic environment, for Democrats to lump unions in with corporate groups as examples of the special interests we need to stand up to?

Second, is Mr. Obama saying that if nominated, he’d be willing to run without support from labor 527s, which might be crucial to the Democrats? If not, how does he avoid having his own current words used against him by the Republican nominee?

Part of what happened here, I think, is that Mr. Obama, looking for a stick with which to beat an opponent who has lately acquired some momentum, either carelessly or cynically failed to think about how his rhetoric would affect the eventual ability of the Democratic nominee, whoever he or she is, to campaign effectively. In this sense, his latest gambit resembles his previous echoing of G.O.P. talking points on Social Security.

Beyond that, the episode illustrates what’s wrong with campaigning on generalities about political transformation and trying to avoid sounding partisan.