By Gideon Long
BBC News, Santiago
The race seems set to go to a second round
Until a few months ago, Chile's presidential election looked like it would be a straightforward and rather uninspiring contest between two seasoned political campaigners - Eduardo Frei and Sebastian Pinera.
Mr Frei is a former president from the governing centre-left coalition, while Mr Pinera is a billionaire businessman hoping to take Chile rightwards.
Both are in their 60s, both are well-known to the electorate and neither could be described as particularly charismatic.
Then, up stepped Marco Enriquez-Ominami. A hyperactive 36-year-old filmmaker with only a few years of political experience under his belt and no party affiliation, no-one gave him a chance when he announced his candidacy.
His bid for success in the first-round ballot on 13 December has blown the campaign wide open.
He trails Mr Frei and Mr Pinera in the opinion polls, and the chances are he won't be Chile's next president.
But by splitting the centre-left he has forced a major re-think within the coalition that has ruled Chile uninterrupted for 20 years.
'Out of steam'
MEO - as he has become known for the sake of brevity - is from a deeply politicised family.
His father, Miguel Enriquez, was the closest thing Chile has to a Che Guevara - a rebel leader who died in a gun battle with the forces of late General Augusto Pinochet in 1974.
Marco was one year old at the time, and already in exile. He and his mother had fled Pinochet's coup the previous year to live in Paris.
It was only in 1986 that MEO returned to Chile. For years, he says, he refused to sing the Chilean national anthem because he associated it with the state that killed his father.
"I adopted Chile and Chile adopted me," he says.
Having made his peace with his country of birth, Mr Enriquez-Ominami now wants to change it.
Like Barack Obama in the US, he has focussed his election campaign on the need for renewal.
He says the ruling centre-left coalition, the Concertacion, has run out of steam after 20 years in power, and Chile needs a new constitution and a new electoral system. Both date from the years of military rule under Pinochet.
Mr Enriquez-Ominami has also challenged the established candidates on social issues like gay marriage and abortion, issues which - in a country regarded as among the most conservative and Catholic in Latin America - are seldom discussed during election campaigns.
"If you look at what the candidates are actually saying on these issues, there's not much to choose between them," says Rodrigo Alvarez, an analyst at the Latin American School of Social Sciences in Santiago. "But the difference is that MEO just seems far more comfortable talking about them than either Frei or Pinera."
"Frei is a Christian Democrat whose party has traditionally opposed abortion and Pinera relies on the support of the conservative Catholic right. There's nothing more awkward than the sight of Pinera talking about gay rights."
Nevertheless, it is Mr Pinera who leads the opinions polls heading into the vote, and many Chileans feel it is he - and not MEO - who represents the change the country needs.
A successful entrepreneur who owns one of Chile's main television stations and a large stake in its best-known football club Colo-Colo, he has been likened to Italian Prime Minister Silvio Berlusconi, a comparison he shies away from.
If he wins, it would mark the first time that a right-leaning government has ruled Chile since Pinochet stepped down in 1990, and the first time the conservatives have assumed power via the ballot box since 1958.
Mr Pinera is campaigning on a tough law-and-order ticket and is promising to use his business know-how to rekindle the economy and generate employment.
His election pledges are bold - an annual economic growth rate of 6% over the next four years and a million new jobs.
The one candidate who is not promising change is Mr Frei, who was president from 1994-2000.
President Bachelet is in the final year of her term
He says the country needs another four years of centre-left rule to cement the social policies of outgoing president Michelle Bachelet, whose success has cast a long shadow over the election.
Mrs Bachelet is tremendously popular - an 83% approval rating according to the most recent reliable poll - and many Chileans would love her to stay on but, under the constitution, she cannot.
She has endorsed Mr Frei, but he is struggling to ride the wave of her popularity, which is based largely on her own personal, warm, easy-going manner.
Most opinion polls have Mr Pinera in the lead.
A recent survey, conducted by the highly-respected Centre of Public Studies, gave him 36% of the vote to Mr Frei's 26% and Mr Enriquez-Ominami's 19%. Jorge Arrate, the fourth candidate in the race, is on 5%, while 14% of those questioned said they were undecided or would spoil their ballot papers.
At the very least, Mr Pinera seems certain to sail through the first ballot, and the big question is who would then join him in the run-off on 17 January: Mr Frei or MEO?
Much will depend on that large swathe of undecided voters.
"In previous elections in Chile it was always pretty easy to predict how those undecided voters would vote - half of them would go left and half would go right," said Guillermo Holzmann, a political science professor at the University of Chile.
"This time things are much more fluid."