President Mauricio Macri suffered a crushing defeat as Argentines voted in party primaries on Sunday ahead of October's general election.
Given that all of the recession-hit South American country's major parties have already chosen their presidential candidates, the primaries effectively served as a nationwide pre-election opinion poll.
Center-left nominee Alberto Fernandez led by around 15 points after partial results were revealed as center-right, pro-business Macri admitted it had been "a bad election."
The first round of the presidential election will be held on October 27, with a run-off -- if needed -- set for November 24.
With 87 percent of polling station results counted, Fernandez had polled 47.5 percent with Macri on a little more than 32 percent and centrist former finance minister Roberto Lavagna a distant third on just 8.3 percent.
"For those who didn't vote for me, I promise to work hard so they understand me," said Fernandez to thousands of cheering supporters in Buenos Aires.
"We're going to begin a new stage. We always fix the problems that others create."
Macri had been hoping to earn a second mandate, but his chances appear all but over.
If Fernandez was to register the same result in October, he would be president as Argentina's electoral law requires a candidate to gain 45 percent for outright victory, or 40 percent and a lead of at least 10 points over the nearest challenger.
Political analyst Raul Aragon had said before the vote that "it would be very difficult to claw back a difference of five points due to polarization: there's no scope to claim votes."
Macri faces the unenviable prospect of having to somehow find three times as many.
- 'It hurts' -
"We've had a bad election and that forces us to redouble our efforts from tomorrow," said Macri, whose popularity has plunged since last year's currency crisis and the much-criticized $56 billion bail-out loan he secured from the International Monetary Fund.
"It hurts that we haven't had the support we'd hoped for," he added.
Fernandez had topped the most recent polls, boosted by his choice of running mate -- the corruption-tainted but still-popular former president Cristina Kirchner.
Around three-quarters of Argentina's 34 million eligible voters turned out for what was an obligatory vote, Interior Minister Rogelio Frigerio said.
Fernandez was a surprise nominee for the Peronist movement as Kirchner -- already president from 2007-15 -- was expected to run again.
However, she is embroiled in a dozen corruption investigations, the first of which has already gone to trial, and is a controversial figure.
Adriana Lucheti, 60, who works in sanitation, said she had voted for Macri "so we can keep changing the 12 years of destruction from the previous governments that stole everything."
Kirchner's eight-year reign followed her late husband Nestor's single term as president.
- Inflation and poverty -
Argentina is currently in a recession and posted 22 percent inflation for the first half of the year -- one of the highest rates in the world.
Poverty now affects 32 percent of the population.
Backed by the IMF, Macri has initiated an austerity plan that is deeply unpopular among ordinary Argentines, who have seen their spending power plummet.
The peso lost half of its value against the dollar last year.
The Buenos Aires stock exchange actually shot up eight percent on Friday amid expectation that Macri would do well in Sunday's vote.
The markets have little faith in Fernandez, who is widely seen as dependent on Kirchner, but that made scant difference to voters.
"I'm going to vote for Fernandez and Kirchner because they're going to create jobs and secure public education. Macri represents unemployment and ignorance," said Josefina Sanchez, a 26-year-old who works for a metallurgy company.