The People's Liberation Front (JVP) said it had come to an agreement with the general that he would be a "common opposition candidate".
Gen Fonseka, who led troops to victory over Tamil Tiger rebels earlier this year, has yet to confirm his candidacy.
President Mahinda Rajapaksa has revealed plans to hold elections early.
However no date has been set.
On Monday, the authorities announced the vote would be held soon and the governing Sri Lanka Freedom Party has now started campaigning for the election.
Around the country blue posters are being plastered on walls, showing the white-clad President Rajapaksa with his arms aloft, and the slogan "heroism serving the country, leadership to develop the country".
President Rajapaksa says he will hold an early election as his government has completed within four years the bulk of its six-year workload.
Gen Fonseka, former chief of defence staff, resigned from his post earlier this month following differences with the government over who should take credit for defeating the rebels.
A spokesman for the main opposition coalition told the BBC that it was still considering whether to endorse the general's candidacy.
The BBC's Charles Haviland in Colombo says the row between the disgruntled general and the government is getting worse.
Gen Fonseka has filed a Supreme Court petition requesting vastly increased security and also seeking leave to stay on in his official residence.
He says he has been reduced to house-hunting and accuses the defence ministry of putting pressure on landlords not to rent to him.
But the military spokesman has countered by saying the government has acceded to all the general's requests.
Our correspondent says the spokesman also accused the general of keeping large numbers of staff and vehicles to which he was not entitled.