The harmful dioxin had been dropped over millions ofacres of South Vietnam during the Vietnam War. By the 1980s, veterans of that war had started complaining of ailments they say were caused by Agent Orange.

Petrou staffed Daschle's investigation on the effects of Agent Orange on soldiers, and advocated for undertaking further medical studies. She worked closely with the military to help get to the bottom of veterans' mysterious illnesses. Petrou continued to follow the issue as a legislative aide when Daschle moved to the Senate in 1987.

"We have gotten a clear sense that the Air Force wants to work with us to get the truth out," Petrou said in 1988.

Petrou's team helped pass Daschle's 1991 Agent Orange Act, which allowed the Veterans Affairs Department to compensate and treat illnesses that were presumed to have been caused by exposure to Agent Orange, and also authorized further independent study.

By then Petrou was Daschle's legislative director, working for Daschle's chief of staff Pete Rouse, now a top Obama adviser.

Private Sector

In 1994, Petrou left Daschle's office and Washington, D.C., to analyze health policy for the North Carolina Health Planning Commission. Later that year, Daschle ran for what he thought would be the Senate majority leader position, but Republicans recaptured the congressional minority and Daschle found himself leading the minority party.Petrou returned to the Hill at the end of 1994.

Anthrax Threat

Petrou continued to work as an adviser to Daschle until June 2001, when she became chief of staff in the senator's personal office. Her portfolio expanded to include defense, foreign policy, Native American issues and telecommunications.

Petrou was chief of staff when, less than a month after the September 11 terror attacks, a letter laced with anthrax was sent to Daschle's Senate office. The anthrax made 20 Daschle staffers ill, but it killed two of the postal workers who had unknowingly handled it.

"The luckiest thing was we saw the powder, unlike the postal workers. That's the critical difference between our people living through it and those two postal workers not living through it," Petrou told Roll Call years later.

Daschle Upset

She remained in Daschle's office until his surprise defeat in his 2004 re-election bid (only the second sitting party leader to lose reelection in history). Daschle left the Senate in January 2005, which left Petrou searching for a job for most of 2005 and 2006.

In November 2006, Petrou landed a spot helping Senator-elect Sheldon Whitehouse (D-R.I.) transition to his new role, and she stayed on as his senior adviser. She left Whitehouse's office in January 2008.

In December 2008, Obama nominated Daschle to be HHS secretary and gave the former Senate majority leader a second role as head of the newly-created White House Office of Health Reform. Daschle installed his longtime aide Mark Childress as HHS chief of staff while he awaited Senate confirmation.

But after the revelation that Daschle owed about $140,000 in unpaid taxes over the use of a car and driver, the former senator withdrew his nomination, leaving a vacuum in Obama's health-policy leadership.

In April 2009, Obama nominated Kansas Gov. Kathleen Sebelius (D) as HHS secretary. When she was confirmed, Petrou replaced Childress, whom she'd known in the Senate, as chief of staff.

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