The Tamil National Alliance (TNA) could tip the balance should parliament be recalled this week to decide between two rivals both claiming the lawful right to head Sri Lanka's government.
Wickremesinghe says his dismissal was illegal as he commands a greater majority in parliament than Mahinda Rajapakse, the former strongman president controversially installed in his place.
The TNA has already said it would vote against Rajapakse in a no-confidence motion when parliament reconvenes.
Both rivals have been marshalling numbers behind the scenes, worrying civil society groups who protested Sunday in the capital Colombo against the horse-trading.
Wickremesinghe's party risks losing legislators to Rajapakse's camp as the days drag by. Some members of his party have already alleged they were offered portfolios and huge amounts of money to switch allegiances.
Six MPs have already switched sides and been granted portfolios under Rajapakse's new administration.
The state-run Sunday Observer, now controlled by Rajapakse-loyalists, defended the change of government but acknowledged the crisis should be resolved.
According to latest counts, Wickremesinghe has 103 MPs from the 225-seat assembly while Rajapakse and Sirisena have 101.
Most of the remaining 21 MPs -- including the TNA -- are set to oppose Rajapakse, observers said.