Voters already made a sharp left turn in January, electing veteran leftwinger and ex-communist Milos Zeman as president after ten years under the staunchly rightwing EU-sceptic Vaclav Klaus.
The Social Democrats (CSSD) are now likely to win the two-day ballot ending Saturday.
They in turn will almost certainly have to rely on Communists to govern, making the far-left a power-broker for the first time since the Velvet Revolution brought down totalitarianism here over two decades ago.
Recent polls have shown the two parties could muster just over 100 seats in the 200-seat parliament, down from earlier forecasts of a 120-seat majority.
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On the Czech Republic's highly fragmented political scene, multi-party coalition governments lacking comfortable majorities are the norm. Smaller parties or MPs sitting as independents are often wooed for backing in crunch votes.
Pehe said the nearly 20-seat slide was rooted in the growing popularity of ANO (YES), a new movement set up by farming tycoon Andrej Babis, dubbed a populist by local media.
Bohuslav Sobotka, the bespectacled 41-year-old Social Democrat leader, renowned for being short on charisma, is nevertheless tipped as the future prime minister.
So far, his party has nixed an outright coalition with the Communists, who make no secret of their nostalgia for totalitarianism. But the fresh-faced Sobotka is still hoping to form a minority government with their tacit support.
"We're looking for partners... And there is often no one but the Communists with whom we could rationally co-operate without giving up CSSD policy," Sobotka said in the run-up to voting.