BUENOS AIRES--Argentine mayor Sergio Massa has an almost 11 point lead over President Cristina Kirchner's list of candidates for mid-term congressional elections in October, according to a recent poll.
Analysts say the elections could decide Mrs. Kirchner's political future amid rumors the two-term president wants to change the constitution to run again in 2015.
Mrs. Kirchner's FPV faction of the ruling Peronist Party has a slim majority in both houses of Congress. But a revolt by Mr. Massa, a popular Peronist mayor, could make it difficult for the FPV to retain the seats it has let alone approach the two thirds majority needed to amend the constitution.
Mr. Massa and his list of candidates would receive 33.7% of the vote in the key electoral district of Buenos Aires Province, followed by Mrs. Kirchner's list headed by Lomas de Zamora mayor Martin Insaurralde with 22.8%, according to a poll by Poliarquia Consultores published Sunday in newspaper La Nacion.
Dissident Peronist politician Francisco de Narvaez trailed third in the province with 12.8%, and a center-left coalition led by Margarita Stolbizer came fourth with 12.1%, La Nacion said.
This year, a third of the Senate and half of the Lower House are in play. Under Argentina's proportional representation system, people vote for lists of candidates drawn up by political parties.
The balance of power in Congress will largely be decided in the densely populated urban belt in Buenos Aires Province, which surrounds the federal capital and is home to about 39% of Argentina's 40 million people.
Mr. Massa enjoys significantly greater name recognition than many of his opponents in the province thanks to his stint as Mrs. Kirchner's cabinet chief and five years as head of Argentina's pension agency.
Mrs. Kirchner has waded into the campaign in hopes of boosting the electoral fortunes of Mr. Insaurralde and the FPV list in the province.
"These aren't the candidates of Cristina, rather of a national populist project that is transforming the country and which I am proud to lead," she said in a speech June 29 in which she formally introduced the FPV's candidates.
The 60-year-old president is widely believed to want to run again in 2015. Mrs. Kirchner has neither confirmed nor denied her electoral intentions, although FPV lawmakers, governors and ministers have publicly called for her reelection.
The poll also found that Mrs. Kirchner still enjoys relatively good approval ratings even after more than a decade of "Kirchnerismo"--the brand of Peronist politics identified with Mrs. Kirchner and her late husband and predecessor Nestor Kirchner, who took office in 2003.
Mrs. Kirchner and her husband frequently enjoyed high approval ratings, driven by Argentina's longest period of economic expansion since World War II, thanks to rising commodity prices, a booming Brazil next door, and heavy spending on social programs and subsidies.
Mrs. Kirchner's approval rating was 39%, and that of her government 46%, according to Poliarquia.
The poll was commissioned by La Nacion and surveyed 1,111 voters in Buenos Aires Province between June 26 and July 5.
Write to Ken Parks at ken.parks@dowjones.com