Presidential candidate John Edwards is caught in a shocking mistress scandal that could wreck his campaign, The NATIONAL ENQUIRER has learned exclusively.

Sources have come forward to charge that the "other woman" previously worked on Edwards' campaign and followed the 54-year-old candidate on trips across the U.S.

A source close to the woman, whose name is being withheld by The NATIONAL ENQUIRER, says that she confessed to having an affair in phone calls and emails, saying that her work with Edwards soon exploded into romance. The shocking allegation — if proven true — could devastate the Democratic hopeful's campaign, especially because John's devoted wife Elizabeth is locked in a desperate battle with breast cancer.

"The affair started about 18 months ago," a friend says the woman confessed to her. "When they met at a bar, sparks flew immediately.

"She never expected it would turn sexual since John is married and is running for President. But it soon did — and she fell for him."

In one bombshell e-mail message provided to The NATIONAL ENQUIRER, the woman confesses to a friend she's "in love with John," but it's "difficult because he is married and has kids."

In another e-mail, she writes: "Last night and this a.m., he actually has amazed me. He is a great man. My heart is loud and my head is silenced."

Disclosed her friend: "She initially confided in a few of her closest pals that she was sleeping with 'a married man named John.'

"It became clear the married man was John Edwards. They got together whenever they could, mostly at hotels where Edwards and his campaign staff stayed."

The woman later spelled it out in a phone call to her pal and talked openly about having an affair with Edwards.

The friend says the woman told her that she "had a crush on John. One thing led to another, and they soon ended up in bed together. But she knew there was no way he was going to leave Elizabeth, a wife battling cancer."

The ex-Senator's wife was diagnosed with breast cancer in 2004. This past March, the couple announced her cancer had returned, but said his campaign would continue.

His spokesman said that allegations Edwards had an affair are "false, absolute nonsense."

The woman's friend told The NATIONAL ENQUIRER: "As the affair went on, she said that both she and John began to feel real guilty and they decided to end it."

The ENQUIRER made exhaustive but unsuccessful attempts to reach the woman for comment regarding this article but she would not return phone calls or emails or come to the door of the house where she is staying.