Argentines will set up a mid-term election standoff between President Mauricio Macri and their fiery leftist former leader Cristina Kirchner in primary votes on Sunday.
The compulsory votes are a formality to officially name the candidates teed up by their parties to fight the October 22 legislative elections.
They are a test for Macri's liberal economic reforms in the poverty-stricken country, two years into his term.
Kirchner, 64, dominated Argentine politics for years alongside her late husband before leaving office in 2015.
Now she is running for senator despite facing corruption charges which she says are politically motivated.
She is looking to beat candidates from Macri's Cambiemos party to win the key seat for Buenos Aires province.
A seat in the Senate would spare her jail if convicted of corruption. Her supporters hope it could also pave the way for a fresh presidential bid in 2019.
Her campaign is a headache for conservative president Macri, 58.
He says his budget cuts are strengthening the economy but Kirchner and other critics say they are hurting the poor.
A third of Argentines are living in poverty, according to a study by Argentina Catholic University.
Macri lacks a majority in the legislature, where he has had to cobble together support for his reforms.
Through his Cambiemos candidates, "Macri is looking to gain strength to ensure he can govern," sociologist Ricardo Rouvier told AFP.
"The opposition wants to send a message to the government that it is doing things badly."