Former Bulgarian Prime Minister Boyko Borissov's Citizens for European Development of Bulgaria (GERB) party is leading in Sunday's snap parliamentary election with 32.8 percent of the votes, the exit poll showed.
Besides BSP and GERB, four other political parties and coalitions will pass the 4% threshold to enter parliament, making it hard to predict what the composition of the next cabinet will be, public TV broadcaster BNT reported, quoting exit polls by Alpha Research polling agency.
The poll showed that the United Patriots party with 8.8 percent, the Movement for Rights and Freedoms with 7.8 percent and the Volya party with 4.8 percent of the votes all passed the 4-percent threshold to be represented in the parliament.
Direct BSP-GERB coalition for now was impossible and any other double coalition was not sufficient, so it would be extremely hard to form a government, he said. "I voted for a stable, predictable and united Bulgaria", Borisov said after casting his ballot, adding: "Bulgarians must decide today who is fit to lead this kind of politics so let them choose". The nationalists said they will stay at the border until Sunday's election to prevent some 50,000 voters from Turkey from entering the country.
In a swift response, his Bulgarian counterpart, Rumen Radev, said his country would not accept democracy lessons from Turkey.
Both times Borisov quit early, first in 2013 after mass protests and then last November after his candidate for the presidency was beaten by an air force general backed by the BSP.
If Ninova can become premier this raises the prospect of North Atlantic Treaty Organisation member Bulgaria, which has long walked a tightrope between East and West, drifting more towards Moscow.
While Bulgaria historically has had strong ties with Moscow, Borisov's GERB party is strongly pro-EU and has supported the bloc's sanctions on Russian Federation over its role in the Ukraine crisis.
Socialist leader Kornelia Ninova wants European Union sanctions against Russian Federation lifted, seeks a bigger role for the state in the economy and has wooed voters with promises of higher salaries and pensions.
Neither of the main contenders is expected to win an outright majority, meaning Bulgaria will most likely have another fragile and diverse coalition government that will struggle to push ahead with reforms to spur growth and combat corruption.
Turkey's support for a new party, Dost, which unlike the MRF fervently backs Turkish President Recep Tayyip Erdogan, has fuelled a spat in recent days.
But analysts have warned that the resulting coalition may not last long.