The number of voters who say they will vote for Bersani's bloc in the Feb. 24-25 parliamentary election is stable, at between 38-39%, and the PD alone is seen getting 32-33 %, the ISPO poll said.
But Berlusconi has boosted his party's standing to between 17-19% from 13-16% at the beginning of December, the poll found.
Bersani's formation is now in a three-way race with the centre-right bloc led by three-time prime minister Silvio Berlusconi's People of Freedom (PDL) party and outgoing Prime Minister Mario Monti's bloc.
Monti's bloc has gained between 14-15 % in polls, from the 10% it was standing at before he entered the race, the poll showed.
Berlusconi bids for Northern League alliance
If Berlusconi can seal an alliance with the Northern League, his coalition could pull in as much as 28% of the vote, ISPO said. The two parties were due to hold talks later on Sunday.
"The PDL has seen its consensus grow, thanks to the ever more frequent presence of Berlusconi on the television screen," said Renato Mannheimer, head of ISPO. Most of the PDL increase came from the large pool of undecided and disillusioned voters, Mannheimer said.
The number of undecided voters, or those who plan to abstain, has fallen below 40%, down from almost 50% a few weeks ago, Mannheimer said. Opinion surveys have shown that up to 50% of the electorate plan to abstain or are undecided in the election.
Both Berlusconi and Monti have made multiple appearances on TV, in Twitter question-and-answer sessions, and in online video interviews over the past week as they seek to close the gap with the centre-left.
Monti unveils new alliance
Meanwhile, Monti on Friday (4 January) unveiled the alliance he will lead into the election. Called "With Monti for Italy", the alliance's logo is a circular symbol with the colours of the Italian flag and Monti's surname in the centre.
The new formation would win 9%, the poll said, and is drawing votes mostly from the centre-left and the previously undecided, according to Mannheimer.
"Most analysts see it as improbable that, as things now stand, the coalition led by Monti can win more than 20%," he wrote.
But if Berlusconi and the Northern League run together, the complexities of the electoral law might make a post-election alliance with Monti key to giving Bersani a stable majority in the Senate, Mannheimer said.
"I hope that (the new alliance) helps improve politics, and that it renews the interest of those Italians who had turned their back on politics, involving them actively again in public affairs," Monti said.
The anti-establishment 5-Star Movement, led by comic Beppe Grillo, dropped to 13-14% from 17-19% a month ago, the poll showed.