Explainer-What you need to know about Argentina's presidential election run-off

By Maximilian Heath

BUENOS AIRES (Reuters) - Argentina will vote in a run-off presidential election on Sunday, with Peronist Economy Minister Sergio Massa taking on libertarian outsider Javier Milei to determine the future of the region's second largest economy.

Massa or Milei will replace outgoing center-left President Alberto Fernandez, also of the Peronist movement founded by former Juan Peron and his wife "Evita" that has been Argentina's main political force for decades.

The winner takes office on Dec. 10.

WHY SHOULD WE CARE?

Argentina, a nation of some 45 million people, is one of the world's main food suppliers, with major exports of soy, corn, wheat and beef. It has one of the largest reserves of electric vehicle battery lithium, and huge shale gas and oil potential.

The country, struggling with economic crisis, is the International Monetary Fund's (IMF) largest debtor, owes billions of dollars to international bondholders and to China, and is a political bellweather in Latin America.

WHO ARE THE CANDIDATES?

Milei is a 53-year-old economist running for the libertarian La Libertad Avanza bloc, who has drawn comparisons with former U.S. President Donald Trump and Brazilian former leader Jair Bolsonaro for his abrasive and theatrical campaign style.

Massa, the current government's wheeler-dealer 51-year-old economy chief, represents the ruling Union por la Patria (UP) coalition. He is seen as a pragmatist within Peronism, which has helped him win over more moderate votes.

Massa pledges to protect the social safety net while Milei wants to "chainsaw" through a system that has left the country in its worst economic crisis in decades, with inflation at 143% and two fifths of people in poverty.

TIGHT BATTLE LOOMS

Massa goes into the second round with some momentum after unexpectedly winning the first round vote in October, when he got 37% to Milei's 30%, overturning pollster predictions.

However, Milei could pick up more middle ground votes after he since got the backing of conservative Patricia Bullrich, the third place finisher from the first round who had 6.3 million votes, some 24%. Not all those though will go to Milei.

Juan Schiaretti, a Peronist but outside the main coalition, got nearly 7% in the first round and could also play an important kingmaker role, though he has been critical of Massa.

Pollsters predict a close race, with some favoring Massa and others Milei. However, they were caught cold by the first round result and the open primary vote in August.

WHAT'S AT STAKE?

Argentina's election comes at a time of major economic uncertainty. Milei wants to dollarize the economy and cut the size of government. Massa would stick with the peso and try to bolster the labor market and growth.

Any new government needs to resuscitate an economy facing triple-digit inflation, negative net foreign currency reserves, and a sliding currency. Meanwhile, a $44 billion IMF program is creaking. Investors and bondholders are watching.

The election of members of the two-tier Congress is already set, with whoever wins the presidency facing a fragmented legislature split in three between the Peronists, the main conservative bloc, and Milei's libertarian coalition.

WHEN AND WHERE?

Voting centers will be open from 8.00 a.m. local time (1100 GMT) and close at 6.00 p.m. on Sunday. The first official results are expected from around 9.00 p.m. local time. In theory there is a blackout on results or exit polls until then, though that has been broken by some local media in recent ballots.

Voting is mandatory for people between 18 and 70, and optional for those between 16 and 17 and for those over 70. The registry for the 2023 election is 35.4 million people and Argentines abroad can vote.

In the runoff, the candidate who obtains the greatest number of total votes wins and would govern for a four-year term.

(Reporting by Maximilian Heath; Editing by Adam Jourdan and Andrew Cawthorne)