With around 34% of ballots counted the democratic alliance will have around 103 seats in the 250-seat parliament, with Tomislav Nikolic's Radical Party expected to get 77 seats garnering 28.6% of the vote.
Tadic proclaimed victory Sunday saying it was a vote "for a European Serbia."
However, Nikolic's nationalist party is likely to join forces with outgoing prime minister Vojislav Kostunica's Democratic Party with 30 seats (13.5%) to challenge Tadic's push towards EU membership.
Nikolic described Tadic's victory declaration as premature and said: "By tomorrow we will have started talks, initially, with Kostunica...we can make a coalition."
Both parties are expected to try and seek an alliance with the Socialists and Liberal Democrats, which garnered 20 and 13 seats, respectively.
Serbia's government collapsed early in March when nationalist Prime Minister Vojislav Kostunica refused to govern alongside the Democratic Party, led by President Tadic, who continued to pursue controversial EU membership, despite EU recognition of Kosovo's independence.
Tadic earlier said the election was a new chance for Serbia to reinforce the country's economic perspectives through the process of integration with Europe while Kostunica said Serbia's parliamentary polls would give Serbs the chance to make it clear that Belgrade would only integrate into Europe if Kosovo was part of the country.
Kosovo, with a 90% ethnic-Albanian majority, has been formally recognized as a sovereign state by over 35 countries including the U.S. and most EU members since it proclaimed its independence from Serbia on February 17.
Russia, Serbia's long-time ally and a veto-wielding member of the UN Security Council, has refused to recognize Kosovo's independence.
