Matteo Salvini, hardline interior minister and head of the anti-immigrant League, has seen his party's popularity shoot up on an “Italians first” campaign which has left the M5S and its leader Luigi Di Maio flagging in the polls.
With a fracturing coalition, the League could move to dump its partner and call snap national elections if a swell in European parliamentary votes puts enough wind in its sails.
“Should the results (Sunday) confirm the current level of support for right-wing parties, Salvini will probably be tempted to pull the plug sooner rather than later,” Agnese Ortolani of the Economist Intelligence Unit told AFP.
But she added: “The most likely scenario is some form of government crisis late this year or early in 2020.”
The March 2018 general election in the eurozone's third largest economy saw the League take home just 17 percent of the vote, while the M5S — which set itself up as the honest, environmentally-friendly alternative to a corrupt old political guard — pocketed over 32 percent.
But in the EU elections the last opinion polls before a pre-vote media blackout showed the League boasting over 32 percent, while the share of the politically inexperienced M5S had plummeted to around 20 percent.
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'Monday morning crisis?'
“For most Italians, the EU vote is being seen as part two of last year's general election,” polling expert Antonio Noto said.
The M5S has upped its attacks on the League in recent weeks, taking advantage of a series of embarrassing corruption scandals involving the far-right party, and has seen small gains over the past few days, according to Italian media.
M5S head Di Maio raised the question this week: “Is the League asking for votes for the European elections, or to provoke a government crisis on Monday morning?”
Salvini was quick to retort: “I'm asking for votes to change Europe, not settle scores in Italy.”
Giuseppe Conte — the purportedly independent prime minister agreed upon by deputy prime ministers Salvini and Di Maio — has also attempted to downplay the risk. Conte said the fighting was just pre-election tub thumping and accused the national media of “seeing a crisis behind every argument”.
All three leaders have repeatedly insisted the government will finish out its five-year term. But the fact remains that a strong League result — over 30 percent — could see Salvini tempted to ditch the M5S for the far-right Brothers of Italy, or a fresh alliance with the party's historic partner, billionaire Silvio Berlusconi's centre-right Forza Italia.
“Now that the government has implemented its flagship spending measures (mainly, universal income and steps to facilitate early retirement), the League will be freer to consider engineering a return to the polls… and try to return to power at the head of a more cohesive coalition of right-wing parties,” Eurasia Group analyst Federico Santi said.
Salvini however may be reluctant to end the coalition for one reason: he is currently ruling the political roost.
“From the League's standpoint, the ageing former statesman (Berlusconi) could turn out to be a liability, and it is not clear how this arrangement would be preferable to the existing one, which has worked well for the League — at least until now,” Santi said.
Rabobank economist Michiel van der Veen agrees: “In government Salvini is clearly dominating Five Star, who are relatively inexperienced when it comes to governing.
“Berlusconi, on the other hand, is a seasoned politician, so a strong presence of Forza Italia in a future coalition would make it harder for Salvini to push his own agenda.”
What's clear is that any fresh political upheaval in Italy, which has had 62 governments since the end of World War II, would spook investors and international markets.
“A government split-up will delay measures to address poor government finances and lead to a further increase of the (already mammoth) government debt,” van der Veen told AFP.
By AFP's Ella IDE and Ljubomir Milasin