The SPS and the DS were the main partners in the previous ruling coalition, which also included Mladjan Dinkic's United Regions of Serbia (URS), which is expected to be part of the new government together with the SPS and the SNS.
Tadic said that Dacic would not have been given the PM's position in negotiations with the DS because the DS did not think that Dacic's party would run the state well enough at this moment.
This does not mean that Dacic does not have the legitimate right to demand to be PM, said Tadic said.
"We have democracy... and now we have a new circumstance. We'll see what the consequences of today's decisions are," he added.
Giving Dacic the mandate to form the new government, Nikolic said the government would comprise the SPS, the SNS, of which Nikolic was president until recently, and the URS, adding that some other parliamentary parties might join in the time ahead.