The moderate Tamil National Alliance (TNA) agreed to back Fonseka's campaign after he signed a three-page programme that he hopes to implement in the island's former war zone, a party official said.
"We have decided to support General Fonseka, but a formal announcement will be made on Tuesday," said the TNA official, who declined to be named. The TNA has 21 seats in Sri Lanka's 225-member parliament.
In his letter, a copy of which was seen by AFP on Monday, Fonseka promised to free Tamil detainees within a month if no charges had been brought against them.
The former military chief, who is credited with leading the military drive against the separatist Tamil Tigers, also promised to establish a completely civilian administration.
"Full restoration of all institutions of civil administration from the office of village headman upwards will be free from military, police and political interference," the letter to the TNA said.
The TNA was at one time seen as a proxy of the Tamil Tigers who were crushed in May with the elimination of the rebels' military leadership after 37 years of fighting.Fonseka pledged to end the state of emergency which had been in force since March 1983, largely to deal with Tamil rebels as well as militants among the majority Sinhalese.
The emergency gives sweeping powers to security forces to arrest and detain suspects for long periods without trial. France has led international calls on Sri Lanka to end the emergency after the ethnic conflict ended in May.
Rajapakse called the January poll in a bid to capitalise on the victory over the separatist Liberation Tigers of Tamil Eelam (LTTE) who launched their campaign for a Tamil homeland in 1972.
Fonseka and Rajapakse have been at loggerheads since the end of the conflict. Fonseka quit after accusing the government of sidelining him and falsely suspecting him of trying to stage a coup.