All the evidence suggests that, when the result of Italy's general election is known later on Monday, it will be a deafening – and sensational – Basta! (That's enough!). The publication of polls in the last two weeks of the campaign is banned in Italy. But results circulating on the internet showed an abrupt surge in support for the Five Star Movement (M5S), which wants to force a reboot, not just of Italy's sleazy politics but of its cronyistic society too.

For the first time in 20 years, we have seen an election that has not been hijacked by the sayings and doings of Silvio Berlusconi. Instead, it has been seized by another, equally controversial figure – the M5S's barnstorming frontman, Beppe Grillo.

The latest reported poll results come from surveys carried out a day or two ago. There are good reasons for thinking the M5S's final result will be even better than they suggest.

One is the likelihood that Grillo's massively attended final rally in Rome has generated last-minute momentum. Another is the weather.

Heavy snow has fallen across parts of northern and central Italy. More is forecast for Monday. There have been violent storms in the Mezzogiorno, southern Italy. And they could return.

Unsurprisingly, turnout is down. By midday, it was off by one and a half percentage points. And that favours the M5S.

Its voters are conviction voters. And since the movement is inextricably tied up with the internet (it grew out of Grillo's angry, anti-establishment blog), proportionately fewer of its members are elderly. M5S voters are more determined – and able – to battle their way to a polling station.

A third factor has to do with Silvio Berlusconi, who on Sunday was the target of a topless protest in Milan by the Femen group (a strange way, you might think, of upsetting the host of "bunga-bunga" parties). The media tycoon's campaign tactics have always involved using any means, however outrageous, to make himself the centre of debate.

True to form, on Saturday, meant to be a day of reflection for the voters, he did it again, saying Italy's judges and prosecutors were worse than the mafia. But, like a lot of his recent gambits, this one looked woefully misdirected.

Italy has moved on since the days 12 years ago when he could sweep to power in a landslide. His blatant sexism looks increasingly out of place, as did a remark he made during a television chatshow implying that he approved of the bribery of foreign officials by Italian firms.

Several last-minute soundings put the M5S ahead of Berlusconi's Freedom People (PdL) party. And there was even talk of it overhauling the centre-left Democratic party (PD).

But that would not necessarily bring it the extra seats allotted under Italy's electoral law to ensure the winner in the vote for the lower house, the chamber of deputies, gets a majority. What counts is not the score of each party, but that of each alliance, and the PD, like the PdL – but unlike Grillo's movement – has pacts with smaller parties that should deliver an extra 5-8% of the vote.

In the senate, it is different. There, the bonuses are allotted regionally. To secure them, the M5S would need to top polls in some of the more populous regions that will decide the contest. Even if it fails to do so, it could have an immense impact by making it impossible for either the PD or PdL to get legislation through the senate (which they have to be able to do to govern Italy).

So far, the PD has been assuming it could reach a deal in the upper house with the list put together by Mario Monti, the outgoing prime minister of the non-party government that came to office in November 2011.

But the Monti list's poll ratings have been falling as drastically as the M5S's have been rising. To get into the senate, there is an 8% threshold, and there are growing doubts as to whether the PM's supporters will get over it.