Mrs. Kirchner, 54, the center-left Peronist party candidate and a senator, was leading in early official results over Roberto Lavagna, a former finance minister, and Elisa Carrio, a center-left congresswoman.

With 30 percent of the voting locations reporting, Mrs. Kirchner had 43 percent, ahead of Mr. Lavagna and Ms. Carrio, who each had 20 percent. Mrs. Kirchner needs 45 percent of the vote outright or 40 percent with at least a 10 percentage-point lead to avoid a runoff.

Rival candidates accused her party of ''systemic theft'' of ballots and other irregularities.

Mrs. Kirchner would become the second woman to be elected leader of a South American nation in two years, after Michelle Bachelet, who became Chile's president last year.

Mrs. Kirchner declared victory late Sunday. In a speech, she said she felt a responsibility not only to lead her country, but ''an immense responsibility for my gender.'' She also paid homage to her husband's accomplishments.

Mr. Kirchner, who sat behind her, stood as supporters chanted ''Ole Ole, Ole, Nestor, Nestor!'' He raised her arm.

''She is going to improve the country much more than her husband,'' said Graciela Aballay, 38, who watched Mrs. Kirchner's speech with her 9-year-old daughter, Maria Victoria.

More than anything, Mrs. Kirchner's victory would serve as a referendum on the four years under her husband, who steered Argentina out of its worst economic crisis in 2001, when it defaulted on $80 billion in loans.

Argentina is poised to record a sixth year of growth averaging about 8 percent. It is enjoying higher prices for exports of soybeans, corn and meat, has increased its reserves and reduced unemployment and inflation.

While voters appeared to favor a continuation of Mr. Kirchner's policies, the next president faces the challenge of taming inflation and a looming energy crisis.

Despite approval ratings of more than 60 percent, Mr. Kirchner decided in July not to run for re-election, in what many analysts believe is a strategy to rotate the couple through the Pink House, the presidential palace here, for 12 years. Argentine election law allows a former president to run again after waiting four years on the sidelines.

Mrs. Kirchner grew up in La Plata, once known as ''Eva Peron City,'' the birthplace of the beloved wife and powerful first lady of Gen. Juan Domingo Peron. Mrs. Kirchner was born seven months after Mrs. Peron, known as Evita, died of cancer.

The Kirchners met in law school in La Plata, where they were activists in the Peronist movement. They later moved to Mr. Kirchner's home province of Santa Cruz, in Patagonia, where Mrs. Kirchner was elected a senator before her husband began his political career.

Early in her political career, Mrs. Kirchner was nicknamed ''Queen Cristina'' by other politicians, a reference to her controlling personality. Facing a fractured opposition in the current election, she campaigned lightly, spending much of the past two months traveling in Europe and the United States trying to woo foreign investors and making clear that, if elected, she would seek to improve Argentina's standing abroad.

Argentina under Mr. Kirchner has embraced the notion of regional integration and has benefited from a stronger relationship with Venezuela's president, Hugo Chavez, who agreed to refinance $5 billion of Argentina's debt.

Despite her apparent victory, several rival candidates Sunday reported voter irregularities in some Peronist strongholds. ''Each time a citizen went to vote, the voting authority at the table said there aren't ballots for your party,'' said Patricia Bullrich, the campaign chief for Ms. Carrio. ''They said: 'O.K., you still have to vote. Vote for a blank slate, but you have to vote.'''

Ms. Bullrich singled out La Matanza, an industrial town on the outskirts of Buenos Aires, but also said that ballot theft had occurred throughout the province of Buenos Aires.

Political analysts called the charges exaggerated