To counteract bad publicity, the company has established a "war team" of former Bush and Kerry election workers.
But they are now faced in some areas by groups of traders who are as good as they are at marshalling public support.
Greenwald, who made Out-Foxed, a film about the prejudices of Fox News, particularly during the Iraq war, doesn't pretend to have made a balanced film.
He is angry and it shows. He is making agitprop, not a work of art, but he is not above some artful dodges, such as dramatic music to emphasise his points and fake newspaper headlines to drum his facts home.
There is also a paean of praise for old-style pioneering America of which Dubya would surely approve.
Greenwald does, however, allow television satirists to have their say against Wal-Mart, which at least shows that he is no more savage than others.
This is a film that will inevitably remind us of the criticism of the big supermarkets here, particularly Tesco, which at the moment is intent on taking Asda - and everybody else - to the cleaners.
It has not come before time, and, as they say at Tesco, every little helps.