A Social Democrat-led four-party alliance is predicted to win about 60 out of 151 seats in parliament.
The conservative HDZ party is tipped to be only a few seats behind.
That would leave both seeking support from the centre-right Most party - it was also the kingmaker after the last elections.
The BBC `s Guy Delauney in Zagreb says that the rhetoric has turned ugly in the run-up to the election.
Our correspondent says that Croatia joined the European Union three years ago, but nationalism and neighbour-bashing have become a feature of the debate.
And memories of the Balkan Wars of the 1990s and World War Two have been stirred up as rivals try to bolster support and distract from domestic problems, our correspondent says.
Almost a year of political deadlock has impeded urgently-needed economic reforms, with the economy relying heavily on tourism along the Adriatic coast.
Croatia remains one of the EU`s mostly poorly performing economies despite recent positive developments linked to its EU membership.
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