Bachelet's likely comfortable contest with conservative Evelyn Matthei, 60, actually marks the first time across all of Latin America that a presidential runoff takes between two women.
"There is a chance Bachelet will get record high support and win with 60 percent of the vote," said Marta Lagos, who heads pollster Latinobarometro Chile.
Indeed Bachelet -- a pediatrician by training who was Chile's first woman president from 2006-2010 -- is likely to draw over 66.3 percent of the vote to Matthei's 33.7, the latest Universidad de Santiago-Ipsos poll found.
The 62-year-old former head of UN Women is well aware that she appears to be coasting toward making history decisively -- again.
"I had the honor to be Chile's first woman president, and it will be great honor once again to be the president of every Chilean man and woman," Bachelet told cheering supporters at her closing campaign rally Thursday.
Matthei, an economist and former Labor minister, and Bachelet both grew up as Air Force generals' daughters before taking sharply different political paths. They even knew each other as schoolgirls.
More than 13 million Chileans are eligible to vote Sunday, but turnout is the big unknown.
This year's race marks the first time that voting in a presidential election is voluntary in Chile. In the first round, more than 50 percent of voters did not bother to cast votes.
Because of the election, shops will be closed on the pre-Christmas weekend. Both women are urging people to get out and vote.
"There is nothing there to indicate that lots of people are going to vote," said Lagos.
Voting starts at 8:00 am (1100 GMT) and stretches for 10 hours. Early returns are expected around 2200 GMT.
Bachelet's quest for greater social justice in Latin America's wealthiest per capita country long has been her political calling card.
She has proposed raising taxes -- to raise $8.2 billion. She wants everyone, not just the rich, to have access to free post-secondary education.
Bachelet is also keen to get to work on wide-ranging reforms backed by most Chileans, including a reform of their constitution. It was left over from the era of dictator Augusto Pinochet (1973-1990).
Bachelet also plans to bring Chile in line with a wave of social liberalism sweeping once conservative Latin America, aiming to legalize abortion and start a debate on same-sex marriage.
In her first term, Bachelet reformed the pension system, improved health and social services, and focused on the well-being of Chile's working class and elderly.
Her earlier time as Chilean leader also coincided with a boom in global demand for copper, Chile's top export.
Matthei, facing what looked like impossible odds on Saturday, has focused on trying to improve things for the middle class.
She has slammed Bachelet's leftist ideas as "experiments that have failed in other countries."
Sunday's winner takes office March 11 for a term running through 2018.
In the first round of voting, Bachelet won 46.67 percent of the vote against only 25 percent for Matthei.
Chile has the highest GDP per capita in Latin America -- $22,362 based on purchasing power parity -- but half of its 17 million people make less than $500 per month.
Matthei's program pledges continuity with the administration of outgoing president Sebastian Pinera, with a tougher stance against tax evasion and a better distribution of national resources.