Conservative Argentine President Mauricio Macri suffered a resounding defeat in primary elections on Sunday.
The primary, in which presidential candidates from all parties take part, was won by his left-wing rival, Alberto Fernández, reports BBC.
Mr Macri, whose austerity measures have turned many voters against him, is hoping to win a second term in office in the presidential poll on October 27.
But analysts say his chances of beating Mr Fernández now look very slim.
Voting in the primaries is compulsory and is not restricted to party members but open to all those eligible to cast their ballot in the presidential polls. Whoever wins is therefore seen as a favourite for the presidential polls on 27 October.
The primaries were introduced in 2009 to cut down on the number of candidates running in the presidential elections with contenders having to win a minimum of 1.5 per cent to contest.