When Silvio Berlusconi put his foot in his mouth yet again last week, claiming that kickbacks were a normal part of doing business abroad, Italy waited for a crushing rebuke from his main opponent in this month's election.

But Pier Luigi Bersani is not a man for the cut and thrust of tough political campaigning and, as usual, failed to deliver the killer blow, managing instead the rather limp soundbite: "Enough with bribes, enough with Berlusconi."

It was no better than Bersani's normal service in front of TV cameras, where the head of Italy's centre-left coalition often looks distracted and uncomfortable. But a lot is riding on him becoming a credible rival to the three-time prime minister as millions of Italians, and even foreign governments, look to Bersani to stop a Berlusconi surge, which analysts fear could prompt political paralysis and take Europe and the global economy back to the brink of disaster.

The final polls allowed before the run-in to the elections next Sunday showed Berlusconi's support had risen to only five points behind Bersani's leading coalition, thanks to a skilful blitz on Italy's TV talk shows, where he is never lost for words. "At the very most, Bersani would do OK as the mayor of Bologna," was a choice putdown.

Bersani, 61, a former communist and son of a mechanic, has been a political activist since he organised, as a child, a strike of altar boys at his local church to protest at the priest's use of donation money. A fan of the rock group AC/DC and a philosophy graduate, he is married to a pharmacist from his home town in Emilia Romagna. It is all a far cry from Berlusconi's "bunga-bunga" parties and dalliances with showgirls.

As the leader of the Democratic party, Bersani has found himself accountable to the party's trade union backers, who last year resisted prime minister Mario Monti's attempts to reform the labour market. But he has proved a reformer himself, using his 18 months as a minister in Romano Prodi's 2006 government to cut the cost of using a mobile phone with legislation that challenged the cartel-like tendencies of Italy's operators – exactly the sort of reforms which many people think Monti's technocrat government should have passed more of last year.

But Bersani's drifting speeches, which he peppers with folksy sayings, have become the favourite target of comedians as the election nears. Even Berlusconi has won laughs by doing an impression of Bersani at a rally. Bersani may yet win the Italian parliament's lower house on 25 February, but he could be forced into a senate alliance with Monti to keep Berlusconi at bay.

Both Bersani and Monti promise reform of Italy's sclerotic economy, mixed with an easing up on tax hikes, although the deal could yet be ruined by Bersani's left-wing coalition ally, Nichi Vendola, who loathes Monti's love of austerity and has said he will refuse to be "the poodle" in a coalition.

That has left Bersani stuck in the middle, just as Berlusconi builds consensus by promising to pay back in cash a hated housing tax imposed by Monti last year and grabbed headlines by signing Mario Balotelli to his AC Milan side, with the former Manchester City striker making an immediate impact on the pitch.

Bersani and Monti will be grateful that television airtime for politicians is rationed in the runup to voting next weekend, while the Pope's resignation and a popular TV song contest have further reduced Berlusconi's chances to make shock promises on air.

But one politician who is continuing to pack piazzas up and down Italy could prove an equal threat. The comic Beppe Grillo, who rails against corrupt politicians and is profiting from a spate of big company scandals, already outpolls Monti, with 16%. He is not standing in the election, but the Five Star Movement party he co-founded is expected to fill about 10% of the seats in both the chamber of deputies and the senate.

Grillo was convicted after a 1980 traffic accident for manslaughter, which prohibits him, under his party's rules, from running for the top office. He said that his candidates – who have never served in parliament – were busy studying good governance before heading for the corridors of power, where they are expected to strip away encrusted parliamentary privileges.

"Grillini" could further weaken Bersani in the senate and obstruct lower house bills, increasing fears of a numerically weak Bersani-Monti coalition government collapsing within months.

This week, Grillo has planned a rally in Piazza San Giovanni in Rome, reportedly upsetting centre-left politicians who regularly hold rallies there. "The centre-left is now realising we are here, but it's too late," he said.