The president's party, La Libertad Avanza, aims to significantly increase its small minority in Congress to boost investor confidence in Milei's vision and maintain the support of US President Donald Trump, who recently provided Argentina with a hefty financial bailout but has threatened to pull away if Milei does not do well.
"Don't give up because we're halfway there," Milei told supporters at a closing campaign event in the port city of Rosario on Thursday.
"We're on a good path."
Half of Argentina's lower Chamber of Deputies, or 127 seats, as well as a third of the Senate, or 24 seats, are up for election in the midterm vote.
The Peronist opposition movement currently holds the largest minority in both houses, while Milei's relatively new party has only 37 deputies and six senators.
The White House and foreign investors have been impressed by the government's ability to significantly reduce monthly inflation - from 12.8 per cent before Milei's inauguration to 2.1 per cent in September - achieve a fiscal surplus, and enact sweeping deregulation measures.
But Milei's popularity has fallen in recent months due to public frustration with his cuts to public spending and a corruption scandal tied to his sister, who also serves as his chief of staff.
"Milei's adjustment has been done with treachery and cruelty," said Axel Kicillof, the governor of Buenos Aires province, during a closing campaign event for the Peronist opposition coalition on Thursday.
"They enjoy each victim of the cuts."
Political experts say more than 35 per cent of the vote would be a positive outcome for Milei's government and could allow him, through alliances with other parties, to block efforts by opposition lawmakers to overturn his vetoes against laws Milei has said threaten Argentina's fiscal balance.
Milei has said he expects a cabinet shake-up after the election that could include members of the centrist PRO party, a frequent ally of the government in Congress.
On Monday, foreign affairs minister Gerardo Werthein resigned.
The election will be closely watched by the White House.
Trump's potential $US40 billion ($A61 billion) bailout of Argentina includes a signed $US20 billion ($A31 billion) currency swap and a possible $20 billion facility.
Following the results, many analysts predict a devaluation of the peso, which they say has been overvalued to contain inflation.
If Milei's party underperforms, that might result in a sharper foreign-exchange policy adjustment.
