By James Painter
BBC Latin America analyst
For the first time in more than 60 years, Paraguay may well have a president from an opposition party.
The Colorados have been in power since 1947, the longest-serving party in continuous rule anywhere in the world.
The country's ruler for 34 years and military strongman, Alfredo Stroessner, used the Colorados as his personal electoral machine. Most Paraguayans have never known any other governing party.
But now there is a real chance of change, as the Colorados are split with one large faction not supporting their official candidate.
Whoever wins the most votes in Sunday's election becomes president as there is no second round.
The three main candidates make up an unusual trio: a former bishop, the first woman candidate in Paraguay's history, and a former general who served time in prison for trying to stage a coup.
Former bishop Fernando Lugo has attracted the most attention. He heads a coalition called the Patriotic Alliance for Change, made up of several parties and organisations. His running candidate is from the traditional opposition party, the Authentic Liberal Radical Party.
Fernando Lugo advocates land reform
Born in 1951, Mr Lugo became a priest in 1977, and served as a missionary in Ecuador for five years.
In 1992 he was appointed head of the Divine Word order in Paraguay, was ordained a bishop in 1994, and then served for 10 years as the bishop of the poor region of San Pedro.
There his support for landless peasants earned him the reputation of being "the bishop for the poor".
He came to national prominence in March 2006 when he helped lead a big opposition rally in the capital, Asuncion.
He resigned from the priesthood in December that year. But the Vatican refused to accept this, arguing that serving as a priest was a lifetime commitment and instead suspended him from his duties.
The Paraguayan constitution prohibits ministers of any faith from standing as a political candidate.
Mr Lugo is keen to present himself as a moderate. He advocates land reform and other measures to tackle poverty, but has distanced himself from the region's more radical left-wing leaders like President Hugo Chavez of Venezuela and President Evo Morales of Bolivia.
Even though he says he does not consider himself a leftist, Mr Lugo does represent a major challenge to Paraguay's fossilised and unequal social structure.
His most prominent pledge is to renegotiate the terms of the country's two huge hydro-electric projects.
In particular he wants Brazil to pay Paraguay a lot more money for the electricity it buys from their jointly-owned Itaipu dam, the world's biggest hydroelectric plant. He says he will take Brazil to the World Court in The Hague if necessary.
Blanca Ovelar is closing the gap on Mr Lugo, partly as a result of the Colorado Party finally getting its electoral machinery into gear.
Blanca Ovelar would be Paraguay's first woman president
Virtually all state employees are obliged to join the Colorado Party, which can boast a membership of about 1.6 million out of 2.9 million registered Paraguayan voters. The party's primaries in December saw 800,000 members turn out to vote.
Ms Ovelar was a rural teacher, and education minister under President Nicanor Duarte.
She has a short history as a party activist and has been hampered by the lack of support from the Colorado faction led by Luis Castiglioni, whom she defeated in the primaries marred by allegations of fraud.
She is also seen as too close to Mr Duarte, who is widely unpopular, and has suffered from the government's poor handling of outbreaks of yellow fever and dengue in recent months.
Ms Ovelar has even distanced herself from the Stroessner dictatorship, saying her father was persecuted by him.
This has not gone down well with some of the Colorado diehards who still view the Stroessner years as a golden age.
If she were to win, she would become the country's first woman president. Three of the four Southern Cone countries, Chile, Argentina and Paraguay, would have female leaders.
Women's groups in Paraguay do not generally regard her as a strong advocate for women's causes.
Former general Lino Oviedo, 64, is the candidate for the UNACE party.
He has had a controversial past: in 1998 he was the Colorado party nominee, but was forced to stand down after being given a prison sentence for an alleged coup attempt two years earlier.
Lino Oviedo has a chequered and colourful political past
He fled the country in 1999 after he was accused of masterminding the assassination of the then vice-president.
When he returned in June 2004, he was imprisoned for the coup attempt, but the Supreme Court overturned his sentence late last year amid accusations that it was a deliberate government ploy to split the opposition vote.
Recent polls suggest that Mr Oviedo is also running Mr Lugo close. He is popular among some sections of the rural and urban poor, and usually speaks to them in Paraguay's second language, Guarani.
He is short on concrete policy proposals, but is generally considered pro-Brazilian, and a conservative on social issues.