Nestor Kircher lost in what was a Peronist heartland
Argentine President Cristina Fernandez is on course for a stinging defeat in mid-term legislative elections, with her party losing control of Congress.
With more than two-thirds of votes counted, results suggest the governing Peronist party has lost its majority in the Chamber of Deputies and the Senate.
Ms Fernandez' husband, former president Nestor Kirchner, also lost his high-profile race for a congressional seat.
Sunday's elections were seen as a test of the couple's popularity.
Cristina Fernandez succeeded her husband as president in 2007, but has seen her approval ratings decline in the face of growing economic problems and rising crime rates.
With most of the votes counted in the populous province of Buenos Aires, Mr Kirchner conceded defeat to a dissident Peronist, wealthy businessman Francisco de Narvaez.
ARGENTINE MID-TERM ELECTIONS
Brought forward from 25 October to 28 June
Half of the 256 Chamber of Deputies seats at stake: four-year term
Third of the 72-seat Senate chosen: six-year term
"We have lost by one-and-a-half or two points and we have no problem recognising it," he said.
The loss of the province, which has always been a Peronist heartland, signals a huge political defeat for the Kirchners, the BBC's Candace Piette in Buenos Aires says.
The likely scenario now is a power struggle within the Peronist party in the run-up to the 2011 presidential elections, our correspondent adds.
President Fernandez's term runs until 2011
President Fernandez had brought forward the date of the election, arguing that this would allow Argentines to unite to face the global economic crisis.
But critics say it was a ploy to squeeze in an election victory before the economic recession set in, our correspondent reports.
The elections took place against a backdrop of deep economic problems, and amid complaints of government incompetence.
Consumer spending has slumped, and crime and poverty are more visible.
Many Argentines have been shifting their savings into dollars and sending it offshore, uncertain of the government's ability to deal with their economic woes, our correspondent says.
A damaging row between the Kirchners and the country's powerful agricultural sector over taxation has added to people's concerns.
Mr Kirchner was president between 2003 and 2007 as Argentines enjoyed an economic rebound and a surge in employment. He is widely considered to be the main power-broker behind his wife's administration.