The mid-term elections will determine how much control left-leaning Cristina Fernandez de Kirchner will have during the final two years of her presidency.
Cristina Fernandez de Kirchner has been out of action during the campaign following emergency brain surgery earlier in October.
She is currently constitutionally barred from running for a third term in office.
Many see Sunday’s elections as the start of the race to replace her.
Voters will choose 127 members of the 257-strong Chamber of Deputies and a third of the Senate’s 72 members.
Correspondents say that if her Front for Victory (FPV) party fares badly, it may signal an end to more than a decade of Kirchner family rule in Argentina.
Cristina Fernandez de Kirchner’s late husband Nestor ran the country between 2003 and 2007, when he was succeeded as president by his wife. She was then elected for a second term in 2011.
Although her party is still strong on a nationwide level, high inflation, insecurity and currency controls have weakened its support among middle class families.
Former cabinet chief Sergio Massa is ahead of her candidate Martin Insaurralde in the key province of Buenos Aires.
Sergio Massa, currently mayor of the tourist town of Tigre, says he wants to put an end to Kirchner family rule when he runs for president in two years.
He was elected mayor as a member of the Front for Victory, but left the party in June 2013 to establish the Renewal Front.
Currently, the constitution limits the number of consecutive presidential terms to two, and a two-thirds majority in both houses of parliament would be needed to amend it.
The August primaries indicated that Cristina Fernandez de Kirchner may struggle to keep control of Congress, let alone build up a two-thirds majority.
Cristina Fernandez de Kirchner was told by doctors earlier this month to stop her congressional election campaign after having surgery to remove a blood clot on her brain following a head injury in August.
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