The documentary “Glen Campbell: I’ll Be Me” follows the legendary singer and his family through his Alzheimer’s diagnosis and the rollicking tour that followed. (PCH Films/Volunteers of America)

Glen Campbell: I’ll Be Me” is a tough movie. In 2011, the legendary guitarist and singer was diagnosed with Alzheimer’s disease just as he was releasing his 61st album, the presciently named “Ghost on the Canvas.” Although Campbell, now 78, had already begun experiencing cognitive loss and disorientation, his musical chops and muscle memory were still remarkably vivid. Along with his wife, Kim, and their three children, Campbell decided to tour for the record, an amazing 150-show run that ended in 2012 when he was no longer able to perform.

Filmmaker James Keach went along for the ride, including the appointment at the Mayo Clinic when Kim and Glen Campbell first received the diagnosis. With sometimes startling candor, Keach chronicles not just the heartening example Campbell presents of fending off the worst predations of Alzheimer’s by staying active and doing what he loves, but the “bad days,” when he lashes out with anger and paranoia, or instinctively licks his dinner plate clean like a toddler.

Viewers may have some misgivings about the degree to which “I’ll Be Me” shows Campbell at his most intellectually and physically vulnerable. But that’s the point of a film that operates less as a conventional biopic or music documentary than as an extended public service announcement about the ravages of Alzheimer’s and the pressing need for federal research dollars. In between soaring performances of such classics as “Wichita Lineman,” “Gentle on My Mind” and “Rhinestone Cowboy” at the 2012 Grammy Awards (where Campbell received a lifetime achievement award), “I’ll Be Me” includes dozens of testimonials from the singer’s colleagues, some of whom — including Bruce Springsteen and Brad Paisley — talk about their firsthand experiences with the illness. Kathy Mattea is especially moving as she recalls massaging her mother’s feet and “pray[ing] for grace.”

As commendable as Keach’s mission is, “I’ll Be Me” winds up paying short shrift to Campbell’s early career, especially his days as a session musician with the Los Angeles studio team the Wrecking Crew. That part of his life is addressed in a whirlwind montage that begins the film (and the cats show up later to accompany Campbell in a moving rendition of “I’m Not Gonna Miss You”). As difficult as it is to witness Campbell’s struggles, especially when he forgets the names of his wife and children, “I’ll Be Me” is an elevating experience, inviting the audience to bear witness to Campbell’s courage, humor and spiritual strength. His story may make for a tough movie, but it’s an important and triumphant one, as well.

★ ★ ½

PG. At Regal Ballston Common Stadium 12 and AMC Loews Rio Cinemas 18. Contains thematic elements and brief profanity. 104 minutes.