WASHINGTON (AP) -- Democratic Rep. David Wu of Oregon resigned his seat late Wednesday, making him the fourth member of Congress to quit this year in the wake of a sex scandal.

Wu, 56, already had announced his intention to resign after his hometown paper, The Oregonian, published allegations that he had an unwanted sexual encounter with an 18-year-old woman. In May, the woman left a voice mail message about the encounter at Wu's Portland office. She has not pursued criminal charges.

Wu made his resignation official in a handwritten letter to Oregon Gov. John Kitzhaber sent shortly before midnight Eastern Time.

"With gratitude to the people of Oregon and confidence that our best days are yet to come, I hereby resign as their United States Congressman for the First District of Oregon, effectively immediately," Wu wrote.

The resignation will set off a spirited special election for Wu's congressional seat. Kitzhaber called a primary election on Nov. 8 and a general election on Jan. 31 to choose a replacement for Wu.

Wu said he also notified House Speaker John Boehner, R-Ohio, of his resignation.

In a prepared statement, Wu said that serving in Congress was the greatest honor of his life. The three-paragraph statement made no reference to the allegations that forced him to step down and no apology. He has told staff that his encounter with the woman was consensual.

"Particularly meaningful to me has been working for more and better investments in science and education. Also, I believe that my support for people who struggle for human rights and civil liberties will ultimately bear fruit in a world that is more just and peaceful," Wu said. "However great the honor and engaging the work, there comes a time to hand on the privilege of elected office - and that time has come."

Wu had said he would resign once Congress concluded its work in increasing the debt ceiling. Some state and federal officials were growing impatient and were calling for him to step down as soon as President Barack Obama signed the bill into law Tuesday.

Democratic officials say they are confident they will retain Wu's congressional seat. The Portland-based district leans strongly Democratic.

Republican Rob Cornilles, a sports-business consultant, will hold a news conference Thursday to announce his candidacy. He ran against Wu last year and lost, 55 percent to 42 percent. Labor Commissioner Brad Avakian and state Rep. Brad Witt are running for the Democratic nomination.

Other potential candidates from both parties also are considering the race.

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Associated Press writers Jonathan Cooper in Salem, Ore., and Tim Fought in Portland, Ore., contributed to this report.

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