WASHINGTON — When Nancy Stetson came to Capitol Hill, her boss recalled last week, she was “a young and idealistic doctoral student” from Rhode Island who wanted to test an idea.

In an era of presidential predominance in foreign policy, she asked, couldCongress make a difference?

After long service as a little-known Senate staffer, Stetson can report that the answer is yes. Yes on prodding a major African nation to forsake the path of racial oppression. Yes on reconciliation between the United States and its onetime foes in a small Southeast Asian country. Yes to easing the afflictions of disease upon millions around the globe.

There’s a footnote to the thesis, too: without the likes of Nancy Stetson, Congress could accomplish few such things. So says her boss, Sen. John F. Kerry, D-Mass., who came close to testing these theories from a presidential point of view.

As she departed the Senate after 22 years, Kerry saluted the woman he called his “alter ego” and Stetson recalled how a girl from Main Street, North Scituate, grew up to walk upon the stage of world affairs.

Stetson went to Mary C. Wheeler, as the Providence school was once known, then Wellesley College and postgraduate studies at Columbia University.

Her first port of call on Capitol Hill was her last: the professional staff of the Senate Foreign Relations Committee, where she started under Rhode Island Democrat Claiborne Pell in the early days of the Reagan administration. Stetson learned the craft of legislation, much of it drafted and negotiated behind the scenes by aides. “One of the best things I learned,” she said, was a favorite Pell adage for the bargaining table: “Let the other guy have your way.”

She also tells with affection how the quirky Pell once defused the anger of a group of citizens that threatened a round-the-clock protest at his office. “I’ll make sure to leave the door open so you can use the bathroom,” Stetson recalls he told them.

Stetson saw duty on many legislative fronts: the annual battles over State Department budgets, the nuclear freeze movement and — a bright moment in her career — the drive for U.S. sanctions to shake the apartheid system of South Africa’s government.

Africa was “her first love,” Kerry said in a Senate speech praising his longtime top aide on foreign affairs, whom he inherited from Foreign Relations Committee Chairman Pell.

Stetson was Kerry’s right hand during the former Navy swift boat skipper’s long campaign for diplomatic actions to heal the wounds of the Vietnam War.

A prerequisite for progress was to resolve the concerns of Americans convinced that many missing U.S. fighters might still be held in Vietnam, years after the end of the war. The work was delicate and painstaking. There were remarkable scenes: Sen. John McCain, R-Ariz., the former prisoner of war, encountering his former North Vietnamese interrogator; lines of Americans and Vietnamese working together on excavation sites, sifting the earth for any remains of the U.S. missing. In the end, Kerry was able to settle the matter of the missing in action to the satisfaction of most Americans — and along the way accomplish much healing between the two nations.

A signal moment came during a late-night talk on the roof of the Rex Hotel in Saigon, a “kind of seedy” wartime hangout of Kerry’s. There, Kerry and Stetson and Tommy Vallely, a close friend and Vietnam veteran from Boston, hatched a plan. The framework they conceived led eventually to the crucial legislation, signed into law by President Bill Clinton, that lifted the U.S. sanctions against Vietnam.

Thanks to the subsequent forging of normal relations, the U.S. national consciousness has changed toward Vietnam, Kerry said. Today, “Vietnam is not just a war but a country,” he said, a change that wouldn’t have occurred without Nancy Stetson’s “vision and her perseverance.”

Mr. Clinton’s normalization of U.S. diplomatic ties to Vietnam was the culmination of adventures Stetson could hardly have envisioned as a young scholar from Rhode Island.

“I thought of myself as a kid who got out of school and worked hard and did a good job,” Stetson said. Never did she imagine herself on a Saigon rooftop, brainstorming about the healing of the national wound from the war, or in the Oval Office months later helping to tell a president how it could be done.

“It was a highlight of my life,” she said, to see this harvest of some 15 trips to Vietnam, of her numberless hours of labor on the MIA issues, on the priorities of individual legislators, on the diplomatic spadework in the State Department.

“It also proved my thesis,” Stetson said, recalling the research that first brought her to the capital, “that Congress can make a difference” in world affairs.

Now Stetson is off to test a variation on her thesis at a small policy shop in Cambridge, Mass. The New England Health Care Institute has its sights set on making a difference in the world of medicine — “a field of a million challenges,” in her words. The resources of Stetson’s new office may be modest — a staff of no more than 15 — but her dreams are not. One example: The institute may try to ensure that the 2008 presidential candidates advance proposals to bring affordable health care within the reach of many Americans who lack it.

As Stetson’s old boss wished her well in the new endeavor, he appended a warning that would ring true with any policy worker who ever toiled in the vineyards of Capitol Hill.

“I hope she knows she is not going to escape my badgering e-mails or 3 a.m. phone calls from Baghdad or Amman to mine her thoughts,” Kerry said.