Christofias, who was 1,000 votes behind Kasoulides in last Sunday's first round, has since won the endorsement of three smaller parties that earlier backed Papadopoulos.
If successful, Christofias would be European Union's only communist head of state and Cyprus, the only European country with a communist president, besides ex-Soviet Moldova.
Meanwhile, Kasoulides, 59, has the support of the island's orthodox church and a backing by the island's conservative DISY party.
Both candidates have said they would attempt to broker a deal with Turkish Cypriots to end the conflict keeping Cyprus divided.
Earlier Papadopoulos, the former president, had led Greek Cypriots in voting down a UN reunification plan that was endorsed by Turkish Cypriots in 2004.
That resulted in a divided Cyprus joining the European Union. He was widely blamed for the outcome.
Cyprus suffered an ethnic split and large-scale violence following its independence from Britain in 1960.
Turkey invaded in 1974 following a Greek-inspired coup aimed at uniting it with Greece.
A breakaway Turkish Cypriot state in north Cyprus is recognised only by Ankara, while the Greek Cypriot in south represents the whole island in the EU.
Cyprus has no post of prime minister and the executive power rests mostly with the president, who is elected for a tenure of five years.
However, allegations of internal agreement and power sharing have been brought out by the Greek Cypriot media.
It reported that Christofias had supposedly promised three ministries, including that of foreign affairs, to the DIKO party of Papadopoulos and two ministries to the socialist EDEK party.
Such an arrangement would limit Christofias' freedom over the Cyprus problem as both the parties have a less flexible approach than Christofias' communist party and the rightist DISY party backing him.