You don't have to look hard to find the union label here. It's woven right into the fabric of these 18 performances, the latest collaborative effort from singer, storyteller and social activist Utah Phillips and his more famous disciple and supporter Ani DiFranco.

Long an evangelist for workers' rights and a walking, talking reminder of the battles fought on the union front, Phillips is an avowed pacifist who is nevertheless partial to songs that radiate a fighting spirit or passionately evoke bloody labor struggles waged in the past.

And he's not one to tolerate passive audiences, either. Which is why he prefaces the blunt anthem "Dump the Bosses" with the following advice: "You're invited not just to sing it but to caterwaul it, because this is like street singing, soap box singing." Then along comes the first verse: "Are you cold, forlorn and hungry/ are there lots of things you lack/ is your life made up of misery/ then dump your bosses off your back."

For all his passion and outrage, though, Phillips is never shrill or long-winded. He's capable of great eloquence, as evidenced by his spoken remembrance of Mother Jones, and he's a fine back porch balladeer to boot.

Wisely, DiFranco refrains from butting in. Though she's not exactly a silent partner here, she spends most of her time in the background with Julie Wolf and Jason Mercer, creating a now clangorous, now quaint weave of stringed instruments, keyboards and percussion. She seems keenly aware of the fact that Phillips can be a one-man rally.

Ani DiFranco appears Monday at Wolf Trap with Maceo Parker. To hear a free Sound Bite from Utah Phillips and Ani DiFranco, call Post-Haste at 202/334-9000 and press 8107. (Prince William residents, call 690-4110.)