WASHINGTON — When Democrat David Wu's political history is written, it will contain at least two noteworthy facts — one historic, the other troubling.

The Oregon lawmaker in 1998 became the first Chinese American in Congress. On Tuesday, Wu announced his resignation amid allegations that he had a sexual encounter with the 18-year-old daughter of a longtime friend and campaign donor.

Wu, 56, said he is stepping down so he can take care of his two children as he fights the allegations. He'll leave once Congress and President Obama resolve an impasse over raising the nation's debt ceiling.

The Oregonian reported Friday that a young woman left a distraught message with Wu's Portland office accusing him of what the newspaper described as "aggressive and unwanted sexual behavior."

"The time has come to hand on the privilege of high office," Wu said in a statement. "I cannot care for my family the way I wish while serving in Congress and fighting these very serious allegations."

Wu becomes the fourth member of Congress this year to leave amid a sex scandal. He joins Republicans Chris Lee and John Ensign and Democrat Anthony Weiner on that list.

When the allegations surfaced, Wu was defiant, but said he would not run again in his Portland-based district. He told a friend, Rep. John Larson, D-Conn., that he did "nothing illegal" and believed he would be "vindicated," according to The Oregonian.

Wu, who did not deny he had an encounter with the woman, conceded he exercised poor judgment.

House Minority Leader Nancy Pelosi, D-Calif., formally asked Monday for an ethics investigation of Wu, but she and other top leaders did not call on him to resign. Wu's resignation announcement Tuesday came after Oregon's two Democratic senators, Ron Wyden and Jeff Merkley, called on their House colleague to step down.

"He would have been worse than a lame duck if he stayed," Jim Moore, a political scientist at Pacific University in Forest Grove, Ore., said about Wu. "Seventeen months is a long time to work with people who don't want to work with you."

Wu, who was born in Taiwan, made reference to his ethnicity in his statement. "Rare is the nation in which an immigrant child can become a national political figure," he said.

Gov. John Kitzhaber, a Democrat, will be able to call a special election once Wu's resignation takes effect. Rep. Steve Israel, chairman of the Democratic Congressional Campaign Committee, vowed Oregon's 1st District would remain in the party's hands, as it has since 1975. President Obama easily won the district in 2008, and Wu was elected to a seventh term in November with 55%.

Two Democrats, Oregon Labor Commissioner Brad Avakian and state Rep. Brad Witt, had already announced their candidacies before Wu's latest troubles. Several GOP lawmakers in Oregon are also considering the race.

"Clearly, the Republicans' best chance for the district are without an incumbent in there," Moore said, adding the special election to pick Wu's successor "could be seen as a dry run of the 2012 elections."

Even before the sex allegations had surfaced, Wu had attracted unwanted national headlines. The Oregonian reported in February that Wu's staff demanded the congressman seek psychiatric treatment just days before the 2010 elections because they were concerned about his erratic behavior. Among other things, Wu e-mailed a photo of himself wearing a tiger costume.

The congressman apologized for his behavior, saying he was on medication and getting treatment for undisclosed mental health issues.

Wu's decision to resign represents a dramatic fall for the lawmaker. He graduated from Stanford, briefly attended medical school at Harvard and earned a law degree at Yale. He clerked for a federal judge in Portland and ended up making his home there, working first for a major law firm and then opening up his own practice focusing on tech issues.

While in Congress, Wu developed a mostly centrist voting record but stuck with the Democratic Party on education, health care, abortion and gun control, according to TheAlmanac of American Politics.