Legendary R&B singer Al Green performs at Wolf Trap National Park in Vienna, Va. (Kyle Gustafson/For the Washington Post)

“Let’s Stay Together” turned 40 last year, but Al Green’s hallmark hit took on a second life at the Apollo Theater back in January when President Obama unofficially kicked off his reelection campaign by singing its opening line: “I-I-I’m . . . so in love with you . . . ”

At Wolf Trap on Friday, Green saved “Let’s Stay Together” for his grand finale, proving that, like any successful politician, a great singer knows how to work the room.

Shoulders bouncing in a blazer as white as dental implants, the 66-year-old pressed the proverbial flesh while he crooned, doling out smiles and handshakes to the front row. Earlier, he plucked roses from a pile and tossed them to admirers like paper planes. And for those in the back, he got frisky with the microphone, sending a sequence of amplified smooches across the summer air.

But Green didn’t utter a word about the commander in chief. This show was about God, Al Green and you.

And for the most part, it was about Al Green. For 60 minutes, he peacocked across the stage, teasing out certain phrases and blasting through others, as if dislodging them from a forgotten chamber of the heart. He made Roy Orbison’s “Pretty Woman” — a song dimmed by years of licensing to movie and television soundtracks — feel like it was something he had just penned backstage. Same for “Amazing Grace,” which he began just a micro-decibel louder than a whisper from across the pillow.

His 13-piece band worked hard to match his nimbleness, and three backing vocalists stood out. They were Green’s daughters, and throughout the set, they laid a gorgeous foundation of sibling harmony that gave Dad plenty of room to show off.

He did exactly that early in the night during “Take Me to the River, ” unleashing a cresting, sustained “Iiiiiiiiiiiiiiiiiiiiiiii” — the kind of falsetto that can scrape paint off a ceiling.

If Obama plans to hit a note that impressive at next week’s Democratic National Convention in Charlotte, Mitt Romney better be practicing his Lynyrd Skynyrd guitar solos. (The veteran rock band is expected to perform at this week’s Republican National Convention in Tampa.)

Because everybody knows that Mitt Romney can’t sing.