Nikolic is acting leader of the hard-line nationalist Serbian Radical Party.
Presidential polls take place Sunday amid intense debate that has pitted hard-line nationalists against more moderate politicians.
Five months after terror-conspiracy convictions, prosecutors and the defense still battle over the evidence.
The pro-West president called Saturday's snap polls after being criticized for authoritarianism. With early results giving him 48 percent of the vote, opponents protest fraud.
Monitor correspondents and experts suggest what to watch for in eight international conflicts.