BUENOS AIRES, Argentina (AP) -- President Cristina Fernandez's ruling Front for Victory lost ground in congressional elections Sunday, giving up seats in Argentina's four largest districts and dashing the bloc's hopes that it would could win enough seats to revise the constitution and let her run for a third term.
The president's former Cabinet chief and now political rival, Sergio Massa, gained the most votes nationwide, one of his allies predicted.
It was too early to tell based on exit polling alone whether the government and its allies lost the thin majorities they have used in the lower house and Senate to dominate the country's political agenda. The first official results weren't due until sometime Sunday night.
Juliana di Tullio, a ruling front candidate for the lower house, said the government believed it wouldn't lose majorities in either chamber.
However, Massa's arrival as a deputy in Congress, as a popular rival with a slate of lawmakers claiming to represent the center in politically polarized Argentina, represents a new threat to Fernandez's all-or-nothing style of governing.
And because Fernandez clearly failed to gain the two-thirds majorities in both houses needed to end constitutional term limits, the vote ensured she will be out of office after 2015, marking the beginning of the end of a government that she and her husband, the late President Nestor Kirchner, have led for a decade.
Exit polls suggested the slate led by Massa, the mayor of the wealthy the Tigre municipality where many of Argentina's rich and famous live in gated communities, did better in Buenos Aires province than the slate led by Martin Insaurralde, who was hand-picked by Fernandez to lead the ruling front's slate of candidates.
"Sergio will be the most-voted-for leader in the entire country with this election. This is an overwhelming response by the people to our times," said Dario Giustozzi, who also appeared likely to take a seat in Congress as part of Massi's Renewal Front.
"This is the end of an era, a new space. Now the people have a place where they can be heard," Giustozzi said.
Voting is mandatory in Argentina, and more than 75 percent of the 30 million registered voters cast ballots, Interior Secretary Florencio Randazzo said.
The Kirchners steadily increased the power of the presidency over the last decade, but Congress may now be in a position to reassert itself during the final two years of her term. While Fernandez remains popular nationwide, particularly among poor people who benefit from many government subsidies, her allies can no longer keep rivals in check by floating the idea of a "re-re-election" to a third term.
Fernandez also needs a majority in each house to reach a quorum and push through her agenda, and the ruling bloc already lost some sure votes in the current Congress. With half of the lower house and a third of the Senate up for grabs in Sunday's vote, she might end up losing more.
The results may not become clear until after Dec. 10, when the new Congress is sworn in and their loyalty is tested.
Of 257 seats in the Chamber of Deputies, the president's bloc had 115 official seats, but could depend on only 109 or 110 votes from its own members and needed help from allies to pass legislation. In the Senate, the bloc had 32 seats and could count on allies for a total of 38 votes, barely more than the 37 needed for a majority in the 72-member chamber.
Fernandez, 60, will need to find new ways of dominating opponents and also maintaining the loyalty of politicians within the ruling front, her center-left branch of Argentina's multifaceted and fractious Peronist party.
Before she was diagnosed with a head injury Oct. 6, Fernandez appeared with Insaurralde at every major campaign event, sometimes doing all the talking. But since she was sidelined by skull surgery, she has remained in seclusion, a very unusual situation for a country accustomed to seeing her on television every day. While her doctors say her condition is improving, they ordered her to rest for a month and avoid any stress.
Her vice president, Amado Boudou, is nominally in charge while she recuperates, but even her top ministers have struggled to describe how decisions are being made, and have contradicted each other about whether she's even aware of major developments. Barred by her doctors from flying, she was unable to vote or visit Kirchner's tomb on Sunday, which was the third anniversary of his death from a heart attack.
With no hope of a third term and Boudou's political future clouded by corruption investigations, Fernandez could now spend her last two years struggling to keep rivals in line during an intense succession battle within the always fractious Peronist party.
The short list of potential candidates has always included the governor of Buenos Aires province, Daniel Scioli, and the mayor of the capital of Buenos Aires, Mauricio Macri, both seen as more business-friendly and centrist than Fernandez. With Sunday's results, Massa appears likely to be one of the candidates to beat.
Still, Fernandez has lost ground before, only to recover her strength. After the last mid-term elections, and following Kirchner's death, many predicted she wouldn't survive politically without him. Instead, she consolidated her power, winning back enough allies to extend an economic emergency law enabling her to unilaterally make major financial decisions.
Fearful of losing that power after Sunday's vote, the government and its allies in the current Congress recently extended the "emergency" until the end of 2015.