Candidates sponsored by Argentine opposition leader Sergio Massa won the House of Deputies midterm primary in the politically key province of Buenos Aires, according to an exit poll announced on television channel TN.
In the open nationwide primary on Sunday, opposition parties were running against President Cristina Fernandez's FPV (Frente Para la Victoria) coalition.
More than 70 percent of the 30 million registered voters chose candidates for October's midterm congressional poll, in an election that - for the first time - included voters as young as 16.
Massa, the mayor of the affluent Buenos Aires town of Tigre, is heading his own list of candidates for Congress and is seen as a possible, business-friendly presidential contender in 2015.
Forty percent of Argentina's electorate lives in Buenos Aires.
The result could determine whether President Fernandez can keep governing with virtually no legislative checks on her power.
Analyst Alejandro Catterberg with the Poliarquia polling firm was expecting no more than 35 percent support for Fernandez party's candidates. That would represent a steep drop since she was re-elected with 54 percent of the vote almost two years ago.
Exit polls also showed government-allied candidates lost in the provinces of Cordoba, Santa Fe and Mendoza.
Fernandez, 60, has said she was not thinking about a possible third term, but talk persists that her congressional allies wanted the constitution changed to allow her to run again.
For that to happen, Fernandez would have to increase her control of Congress in October, when half the seats in the lower chamber will be up for grabs along with a third of the Senate.
Fernandez's allies would need a two-thirds majority in both chambers to get debate started on a constitutional change to permit a third term.
"She doesn't have that now and she won't have it after October," Labaqui said. "But if the ruling party gets 40 percent or better at the national level, it could still claim to have a strong enough mandate to pressure the opposition into a bargain allowing the third-term amendment."
Sunday's vote was seen as a mega-opinion poll on Fernandez's heavy-handed trade and economic policies.
An over-valued currency, protectionist trade policies, ever-tightening foreign exchange controls and Fernandez's decision to nationalise Argentina's private pension system and top oil company YPF have upset investors and trade partners.
Argentina's bond investors and those interested in its vast farm and shale oil resources watched the primary for signs that voters may be tiring of Fernandez's approach and are ready for a market-friendly leader in 2015.
On the sidelines is Buenos Aires' popular governor, Daniel Scioli. He is officially allied with Fernandez but could step up to represent the FPV and run for president if her candidates do badly this year.
Scioli is seen as more of a centrist than Fernandez and would be embraced by business leaders.
Argentina's economy is meanwhile expected to grow by about 5 percent this year despite looming fiscal troubles.