Her explanation came in a letter submitted Monday evening to the Senate Judiciary Committee, which also included copies of several rulings, briefs and speeches the judge had not previously given the panel.
GOP senators had requested the material and questioned Sotomayor's membership in the Belizean Grove, a group of prominent professional women, because federal judges are bound by a code that says they shouldn't join any organization that discriminates by race, sex, religion, or nationality.
"I do not believe that my membership in the Belizean Grove violates the Code of Judicial Conduct," Sotomayor wrote. She told senators that the group involves men some of its events and no man she knows of has ever tried to become a member.
Sotomayor's backers noted that the court's only current female justice, Ruth Bader Ginsburg, belongs to the membership-only International Women's Forum, as was former Justice Sandra Day O'Connor, who defended her involvement in all-women's groups during her Senate confirmation hearings.
Even as Republicans delved into Sotomayor's background, a key GOP senator was planning a series of speeches that will criticize Obama and Democrats for their approach to choosing judges, arguing their approach threatens the court system and the rule of law.
Sen. Jeff Sessions of Alabama, the top Judiciary Republican, was expected as early as Tuesday afternoon to begin making that case, with a speech about what the founding fathers considered an ideal justice. He planned to contrast that vision with a growing movement that he says wants judges to be able to impose their politics and personal feelings through their rulings and change the meaning of the Constitution.
Sessions' planned speeches - which don't include any direct criticism of Sotomayor or her record - came to light as Republicans struggle to figure out how to handle the politically charged debate over confirming the woman who would be the first Hispanic on the high court. GOP senators have neither the votes nor the appetite to try to block the judge, and seem loath to criticize her too strongly, for fear they will be tied to prominent conservatives outside the Senate who have called her racist.
The risk is particularly acute for Sessions - himself rejected for a spot on the federal bench more than 20 years ago after allegations surfaced that he made racist remarks and targeted black civil rights leaders as a prosecutor. But he and other Republicans are also being pressed by conservative activists to use the debate over Obama's first Supreme Court nominee to persuade the public that Democrats are threatening the court system.
Meanwhile, Sen. Patrick J. Leahy, D-Vt., the Judiciary Committee chairman, told civil rights leaders and law students that Sotomayor's confirmation was a certainty.
"You better believe we're going to get her confirmed - take that one to the bank," Leahy said during a speech at the University of the District of Columbia law school.
He compared Sotomayor's nomination to that of Thurgood Marshall, the first African American to sit on the court, saying they both faced adversity. He noted that Republicans questioned Marshall at his confirmation hearings about whether he would discriminate against white people - much as they have challenged Sotomayor for saying that she hoped that a "wise Latina" would usually reach a better conclusion than a white male without similar experiences.
Leahy also defended the notion that a judge should take into account the real-world impact of his or her decisions, saying the failures of the court's "conservative activists" including Chief Justice John Roberts to do so had recently resulted in the gutting of key anti-discrimination and civil rights laws.