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The leadership battle has broader implications as the three partners of the PS in the ruling coalition have said they would not work with Janković in government.

If he emerges the winner, the outcome would almost certainly lead to the resignation of Bratušek as PM and consequently the fall of the government.

In such a case, the other coalition partners want a snap poll as soon as possible.

The current uncertainty stems from events in early 2013, when Janković stepped aside to allow Bratušek to take over in order to form a new government with the Social Democrats (SD), Citizen's List (DL) and Pensioners' Party (DeSUS).

The parties had refused to work with Janković after a report from the anti-graft watchdog concluded he had run afoul Slovenia's office-holder integrity legislation.

In spite of his withdrawal, his position was never fully clear, as he had merely suspended his leadership. He soon made it obvious that he continued to harbour leadership aspirations, setting the scene for today's showdown.

Janković maintains that he could emerge the leader and stay on as Ljubljana mayor, allowing Bratušek to continue leading the government, but she has said it is hard to imagine her leading the government without support in her party.

The tussle has seen both campaign hard to secure support from key members of the party in the run-up to the vote, with analysts highlighting the turnout as a key factor in the outcome. A higher turnout - above 800 of 2,500 members - is thought to favour Janković.

Meanwhile, either can still withdraw their bid at the congress itself. Pressure has mounted on Janković to give up his bid in recent days from figures instrumental in the shaping of PS ahead of the 2011 parliamentary elections.

One of them, former President Milan Kučan, has cautioned against a new political crisis in the country in recent days. He said that national interests must be put above party interests.

Janković, who argues that he was instrumental to leading the party to victory in the elections, has so far not shown signs of changing his mind, telling the press earlier this week when "I play, I play to win".