It is ironic, but now that Linda Ronstadt's commercial star is in decline, she is singing far better than she ever did in the past. No longer content to cruise down the safe middle of songs, Ronstadt consistently pushed against the edges of her material at the Merriweather Post Pavilion last night. Not only did she sing with a new fullness and authority, but more important, she twisted key lines out of the arrangement to give songs an interior development they never had before.

Assuming the title role in Elvis Costello's "Party Girl," Ronstadt promised "I would give you anything but time," and then repeated "time" with a ghostly wail that betrayed the hollow promise. She belted out "When Will I Be Loved?" although she ended with an utterly forlorn tag. She sang John Carroll's "Get Closer" with an impatient shudder that gave the song a new eroticism.

Ronstadt sang a nicely understated duet with surprise guest J.D. Souther on "Prisoner in Disguise." Despite a sluggish rhythm section, she shoved her way through rockers like "Back in the USA" and "I Can't Let Go," and her newly confident falsetto provided climactic harmonies in "Blue Bayou" and "That'll Be the Day."