District Attorney Michael Schrunk said the case has numerous problems and isn't appropriate for a criminal prosecution.
Among the difficulties that Schrunk cited: Although the red-haired masseuse said she was terrified of Gore, she also said she called him after their encounter and told him to "dream of redheaded women."
Schrunk also said the woman -- Molly Hagerty -- told the hotel she appreciated the business referrals it had given her, and didn't mention any problems with Gore just two nights earlier.
Gore denied the charges, including under questioning July 22 by local detectives. His aides welcomed the news.
"Mr. Gore unequivocally and emphatically denied this accusation when he first learned of its existence three years ago," spokeswoman Kalee Kreider said in a statement. "He respects and appreciates the thorough and professional work of the Portland authorities and is pleased that this matter has now been resolved."
There were questions about the veracity of the accuser's claims from the start.
Detectives investigated the claims in 2006 and 2009 but decided not to pursue the case because of what they called the accuser's lack of cooperation and erratic behavior. She waited two months after the alleged incident to contact police, then canceled several meetings only to finally meet with officers three years later.
It's still unclear whether she sought compensation to tell her story, and whether she received payment from any publication; news reports indicated she had sought $1 million from the National Enquirer, which broke the story about her accusations against Gore.
The Portland Tribune said it thoroughly investigated her allegations -- including interviewing the accuser -- only to determine it wasn't responsible to move forward with a story. The paper reported that she failed a polygraph test. At least one other news organization also declined to publish a story.