Lorraine Hunt Lieberson with husband Peter. (Courtesy of Peter Lieberson)

Kelley O’Connor can’t finish the song cycle without bracing for sobs.

It has happened seven, maybe, eight times. She’s lost count by now. She’ll be standing center stage in a jewel-toned gown, orchestra behind her, and with an earnest plea, she’ll sing:

“Si volveras o si me dejaras muriendo?”

Will you come back? Will you leave me here, dying?

And then, without fail, the mezzo-soprano will cry, larynx suffering the consequences.

Mezzo-soprano Kelley O'Connor. (Zachary Maxwell Stertz)

Yes, O’Connor will wow, too. She’ll dazzle and lure the audience into a love-rapt stupor when she takes the stage with the National Symphony Orchestra next week to sing Peter Lieberson’s “Neruda Songs” at the Kennedy Center. But even with much practice, when O’Connor, 32, reaches the third song, her eyes will well up in sync with her audience’s tears.

“When it says, ‘Don’t leave me for a week. Don’t leave me for a day. Don’t leave me for an hour. For a minute. For a second,’ ” she trails off. “It’s that intense, almost scary kind of love, when you can’t be away from the person. That’s the one I have a hard time singing.”

Because O’Connor witnessed that kind of love and is now the guardian of it.

In 2008, O’Connor accepted the request to sing “Neruda Songs,” knowing that she would be narrating a love story of two star-crossed lovers torn away from each other too soon. It’s the plotline of every tragic opera, from “Tristan und Isolde” to “Madama Butterfly.” Except that this is not the plotline of the song cycle, but rather the back story, of how and why Lieberson composed “Neruda Songs” for his wife, mezzo-soprano Lorraine Hunt Lieberson.

The contemporary composer chose five sonnets by Chilean poet Pablo Neruda, each depicting a stage in love’s transition from ephemeral to eternal. Lieberson wrote the songs for his wife as a gift, and Hunt Lieberson first sang them with the Los Angeles Symphony Orchestra in May 2005 to critical acclaim.

But when Lieberson wrote the piece, he didn’t know that his wife would sing them only three times or that she would die the following year at age 52. He didn’t know that they would both lose their lives within six years to cancer — hers of the breast, his of the lymphocytes — their New York Times obituaries reading like bookends to a singular life.

Now, O’Connor’s duty, one she calls “a gift,” is to retell this discourse on love after the deaths of both muse and artist.

“When Peter passed, I had to sing the songs a month later,” O’Connor recalled. “That was the hardest performance. On stage, I was thinking, ‘He’s gone, but they’re listening together. They can finally be together.’ ”

The last time a Kennedy Center audience heard “Neruda Songs,” Hunt Lieberson ended the piece with “My love, if I die and you don’t.” She fell into her husband’s arms as he took the stage during the spirited ovation. No one in the audience anticipated their tragic end, and it is unclear whether the intensely private Liebersons knew their fates. But that their love story would survive with the help of a talented stranger — that they could not have known.

O’Connor knew of Peter and Lorraine, though she didn’t call them that at the time. She had idolized Lorraine, whom the New Yorker called “the best mezzo since [Maria] Callas.” And it was a dream when Carnegie Hall’s Ara Guzelimian, now the dean of Juilliard, recommended her to Bernard Haitink, who would conduct “Neruda Songs” with the Chicago Symphony Orchestra. Peter was undergoing chemotherapy and approved her selection haphazardly. He wouldn’t care about her performance until later.

Only months before O’Connor would sing the songs in Chicago, she realized that she’d be following one of the world’s best mezzo-sopranos, singing love songs written by her composer husband.

“I knew it was going to be difficult,” O’Connor recalled, laughing at her own understatement. “Because he had never heard anyone but Lorraine sing them. I called him and asked, ‘Do you want to hear me sing them beforehand?’ ”

Peter had just finished his second round of chemotherapy.

“He said to me, ‘I feel like being someplace warm, so I’m going to be in Hawaii. Why don’t you come, and we’ll sing them there?’ ”

O’Connor, then performing in Australia, hopped on a plane, never one to turn down what she calls “an out-of-this-world” moment. Peter picked her up at the airport. For the next week, they would practice a song every day, using a keyboard that Peter couldn’t really play. “He was not a great pianist; he’d be the first to tell you that,” O’Connor said, laughing.

“In some ways, it was ideal that we were in a paradise situation,” O’Connor said. “We shared meals and cooked together. He’d tell me stories about Lorraine, which was amazing since she was one of my idols.”

And she learned the little things that made them human. They were both Buddhists. They both loved great food. Lorraine adored pink champagne. She loved pink everything — pink walls, pink furniture; it was the color of their home in Santa Fe, N.M.

A pink book was the genesis of “Neruda Songs.” Peter saw the rose cover of Neruda’s “100 Love Sonnets: Cien sonetos de amor” gleaming in the Albuquerque airport bookstore, and he picked it up for her. She spoke Spanish, and he did not, but they read it together. He then decided to compose the work for her, with Lorraine picking out five sonnets for the composition. That would be the only help she gave him.

“He gave it to her completed, without consulting her,” O’Connor said. “That’s a feat in itself. It was such a different compositional voice for him. It doesn’t sound like anything he’d written before.”

In Hawaii, Peter told O’Connor of past and present loves, too, which surprised the singer. Peter’s three daughters, his children from his first marriage, were in Hawaii with them. He was still married to his first wife when he met Lorraine in 1997 at the Santa Fe Opera’s rehearsals for his work “Ashoka’s Dream.” Lorraine was singing the part of Ashoka’s second wife, Triraksha. His first marriage ended in divorce after the performance, a time that his obituary described as a “difficult period of family separation.” Peter married Lorraine in 1999. After Lorraine’s death in 2006, he married Rinchen Lhamo, a longtime friend.

“He really had three great loves of his life,” O’Connor said. “At Peter’s memorial, I sang three of the songs, and his first wife and three daughters were sitting there in the front row. There was so much love in Peter’s life, and these pieces transcend his love story. It could be anyone’s. That’s why people relate to it.”

That one person could know another’s voice and spirit so well moved audiences as well as critics. Tim Page, writing in 2008 for The Washington Post, called the songs “one of the most extraordinarily affecting artistic gifts ever created by one lover to another.”

And O’Connor has since carried on the legacy of the work and the lovers, with Peter attending her first three performances in three cities. He cautioned her not to look around at the audience, perhaps knowing that she’d cry through the most moving parts.

“So instead, I’d find him in the audience and sing Lorraine’s words to him,” she said. “I’m not a very spiritual person, but I definitely felt Lorraine’s presence. Always.”

Peter stopped attending the concerts after her third performance at Carnegie Hall.

“After that performance, he said, ‘They’re yours now, I don’t need to come anymore.’ ” He died soon after, in 2011 at age 64.

Now, other mezzos are singing the song cycle, a bittersweet feat for O’Connor, who, although attached to the songs, has always championed contemporary music and hopes many orchestras will program “Neruda Songs.”

“We can carry on the tradition of this work,” O’Connor said. “That’s the beautiful message of it. Love doesn’t die with the person. It didn’t die with Lorraine. It didn’t die with Peter. It’s still living. The seed was planted, and it just continues.”

“Neruda Songs”

Kelley O’Connor and the National Symphony Orchestra perform Peter Lieberson’s “Neruda Songs” at the Kennedy Center, Oct. 4-6, conducted by Christoph Eschenbach. Program also includes highlights from Tchaikovsky’s “Romeo and Juliet” and the prelude to Wagner’s “Tristan und Isolde.” Tickets are $10- $85. For more info, visit www.kennedy-center.org.