Little more than two months ago, the centre-left led by Pier Luigi Bersani was riding high with an apparently unassailable lead of 10 points against a demoralised centre-right, which looked close to disintegration.
But in December a rejuvenated Berlusconi stormed back into the race with an extraordinary media blitz in which he rushed from political rallies to television studios, adopted a homeless puppy, told off-colour jokes and promised to abolish a hated new property tax.
This week he outraged his opponents by mailing official-looking letters to millions of Italians promising to reimburse payments already made under the levy.
Between December and February 8, when a pre-election ban on publishing opinion polls took effect, the 76-year-old billionaire had halved the gap with the centre-left. The colourless Bersani seems to have been stuck in his tracks, unable or unwilling to respond to the crowd-wooing tactics of his opponent, a born showman.
His mockery of what he calls impossible election stunts by Berlusconi have been low key and delivered in his familiar deadpan voice, failing to hit home with many voters.
Most pollsters nevertheless believe the centre-left is still on course to win by 5-6 points, although a big surge by anti-establishment comedian Beppe Grillo is adding to uncertainty, raising chances of an inconclusive result that could destabilise the euro zone.
The most likely result, pollsters say, is that Bersani will have to form a governing pact with outgoing technocrat Prime Minister Mario Monti, although there are concerns the economics professor's own lacklustre electoral performance will fall short of enough votes for this scenario.
Leading elections expert Roberto D'Alimonte also says polls underestimate Berlusconi's support because people are reluctant to admit they will vote for him, given his reputation for scandal and allegations of sex parties that offend conservative and religious voters who otherwise like his policies.
All this is putting nerves on edge among centre-left supporters, frustrated that Bersani has not led a more dynamic campaign and tormented by memories dating back to the 2006 election which is burned into their collective psyche.
Berlusconi, who learned to work crowds as a singer on a cruise liner on his youth, used the same showmanship to close a 10-point gap then, forcing the left's Romano Prodi to form a weak government that collapsed two years later. Berlusconi returned to power for his fourth term with a record majority.
BERLUSCONI PREDICTS VICTORY
"I have good news. We are ahead and we will win," Berlusconi told ecstatic fans at a rally in Milan on Monday, a claim that cannot be verified because of the pre-election polling blackout.
Bersani, the son of an auto mechanic, is a famously bland if worthy candidate with a good track record as a minister under Prodi. Although his spontaneous remarks, delivered in a distinctive accent from the northern Emilia region, can be sharp and humorous, his scripted speeches are often sleep-inducing.
In contrast the livewire, wisecracking Berlusconi has outclassed both him and Monti in the charisma stakes and shown remarkable energy and dynamism for his age.
In addition, the centre-left appears to have thought that their best tactic was to sit tight on their lead and leave the other candidates to make mistakes in a bitter campaign.
Another mistake seems to have been to leave it until the last minute to roll out Matteo Renzi, the youthful mayor of Florence, a lively orator who mounted a credible challenge to Bersani for the party leadership in December.
The suspicion is that the formerly communist left wing of Bersani's Democratic Party dislike Renzi, who is considered too much to the social democrat right.
However, Renzi has recently made appearances, perhaps reflecting party alarm about the polls, and will share the stage with Bersani at a rally in Palermo later on Wednesday.
"We have to make a final push," Bersani told supporters on Saturday as he heightened the urgency of his tone in the southern city of Lecce. "The right must not be underestimated."
SURPRISE BERLUSCONI RESURGENCE
The resurgence of Berlusconi was a surprise to many.
Weakened by a lurid sex scandal, he was forced from power and replaced by Monti in November 2011 as Italy slid towards a major debt crisis. He spent the next year in the shadows, demoralised and depressed, before surging back in December.
He has been hounded by magistrates for two decades on a string of fraud and corruption charges and is currently on trial for having sex with an underage prostitute at "bunga bunga" parties in his villa outside Milan.
However, Berlusconi, backed by an immense personal fortune and a media empire including three TV channels and a magazine publisher, skilfully exploits his image as a flamboyant joker and lovable rogue that still attracts many ordinary Italians.
The centre-left says it has learned its lessons and will avoid the disunity which tore it apart in the 2008 election.
Then, Prodi was betrayed by members of his unwieldy 11-party coalition, but this time Bersani leads effectively a two-party alliance and leftist SEL party leader Nichi Vendola, the junior partner, has signed a pact to abide by joint decisions.
"The centre-left is no longer the same as it was under Prodi," Bersani said this week. "In Prodi's time, there were a dozen parties in the centre-left.
"I predict our coalition is solid and will last a very, very long time," he told a rally in Calabria in the south.
Prodi, now a U.N. envoy in Africa, told a rally in Milan on Sunday: "This is a different team from the past and it will remain united because it has learned its lesson.
"It is made up of different men."
(Editing by Barry Moody and Alastair Macdonald)