Milei, an economist and former TV pundit, has rattled the political elite with loud, sometimes expletive-ridden critiques of his rivals, along with pledges to shutter the central bank, slash the size of the government and dollarize the economy.

The latest poll from consultancy Analogias shows Milei with 31.1% of the vote, ahead of Massa on 28.1%, with right-wing ex-security minister Patricia Bullrich on 21.2%, a blow for the main conservative opposition, which was once favorite to win.

A second poll from Opinaia gives Milei 35% of the vote, with Massa on 25% and Bullrich at 23%. The polls show Milei's support stronger among men, youth voters and less affluent groups.

"The result of the primaries was shocking, and the result of the general election will be shocking whatever its result is," said Analogias communication director Marina Acosta, citing the race being "colored by political novelty."

The polls suggest the election race will likely go to a head-to-head second round, which would take place in November. A candidate needs 45% of the vote, or at least 40% with a 10-point lead to win outright in the first round.

Milei, who made a name for himself as a "shock jock" TV commentator, has struck a chord with angry Argentine voters grappling with 113% inflation, a cash-strapped government, sagging economy and some four-in-ten people in poverty.

His closest competitor now appears to be Massa, part of the ruling center-left Peronist coalition, seemingly offering two opposing economic models for the embattled country.

"Milei's and Massa's strategies are meant to be polar opposites from the other," said political analyst Julio Burdman from consultancy Observatorio Electoral. "They are doing that quite well, which is letting them both make gains."

A big caveat, however, is that Argentina's pollsters got the primary vote badly wrong and misread the last general election in 2019, meaning the final result could shock once again.