By Gavin Jones
ROME (Reuters) - A month away from a referendum on constitutional reform that could sink the government, Italy's largest opposition party wants to make sure Prime Minister Matteo Renzi is not saved by the votes of Italians abroad.
Luigi Di Maio, one of the most prominent faces of the anti-establishment 5-Star Movement, heads to London on Tuesday to kick off a 10-day campaign against the referendum that will also take him to Madrid, Brussels, Paris and Berlin.
5-Star, whose appeal is based on a drive against the corruption that has tainted Italy's politics for decades, has good reason to be wary of the army of 3 million expatriates who have the right to vote.
The maverick group was the most voted party in Italy at the last national election in 2013, but after the ballots were counted from Italians living abroad it was overtaken by the Democratic Party (PD), allowing it to lead a new government.
Di Maio, a 30-year-old parliamentarian widely expected to be 5-Star's candidate for prime minister at the next election due in 2018, aims to avoid a repeat performance with the referendum.
"We want to explain the risks of this reform to Italians abroad because if 'Yes' wins we will never get rid of the political class that forced them to emigrate," he told Reuters in an interview.
Italians faced with a chronically stagnant economy and a lack of work have been emigrating in growing numbers in recent years. Some 107,000 moved abroad in 2015, according to official data, but the actual figure is believed to be much higher.
Renzi says his plan to reduce drastically the role of the Senate and curb the powers of regional governments will simplify decision-making and ensure stable government.
Opponents say it will actually make the legislative process more complicated and reduce checks and balances.
"This reform will give more power to the people who have brought our country to the state it is in," said Di Maio, whose party is roughly level with the PD according to recent polls.
Earlier this year Renzi repeatedly said he would resign if he lost the Dec. 4 referendum. In the last two months he has declined to confirm that, saying discussion of his own future deflected attention from the merits of the reform.
"If he loses we will ask Renzi to keep his promise and the Italians will ask him to keep his promise," said Di Maio, who is viewed as the moderate face of the movement created by firebrand comedian Beppe Grillo in 2009.
With all the opposition parties lined up against the reform, Renzi faces an uphill struggle. Of 33 opinion polls published in the last month, all but one has put 'No' ahead.
However, with around a quarter of voters still undecided, pollsters say the result remains highly uncertain.
Moreover, the polls do not include Italians living abroad. Pollsters expect less than a third of these to cast a ballot, but they say of those that do, most will probably vote 'Yes'. In a very tight race this could be crucial.
The meticulous Di Maio is likely to have a tough task convincing expats such as Stefano Greco, a 27-year-old from Sicily who has been in London for 15 months doing a post-graduate degree at the London Business School.
"I will vote 'Yes', because living abroad you see even more clearly how slow and bureaucratic Italy is, and my impression is that the reform will help to make things simpler," he said.
The 2013 election was just the latest to turn on Italians abroad. In 2006, it was the foreign vote that gave centre-left leader Romano Prodi his majority in the upper house Senate.
5-Star is not the only party wooing Italy's emigrants.
Reforms Minister Maria Elena Boschi recently toured Argentina, Brazil and Uruguay to win support for 'Yes', while the right-wing Northern League is also sending a senior politician to South America this month to whip up the 'No' vote.
Federico Benini, head of polling agency Winpoll, forecast that around 300,000 Italians abroad would vote, or roughly 10 percent. That compares with 30 percent at the 2013 national election and just 3 percent at the 2014 European election.
Of those that cast a ballot, he said he expected up to 80 percent to back 'Yes' because they follow Italian politics less closely, tend to be less anti-Renzi and see it as broadly positive that the country is trying to reform.
Benini estimated that if 'Yes' is less than one percentage point behind among domestic voters, the result could be overturned by those resident abroad.
Di Maio said 5-Star had its own proposals for amending the constitution, such as taking away parliamentarians' immunity from arrest and removing the provision that Italy must balance its budget each year, which in any case it has never respected.
"We are certainly not against change," he said. "But change is not always good, and by saying no to this reform we are trying to save Italy from a change for the worse."
(This version of the story was refiled to add dropped word in third paragraph)
(Editing by Crispian Balmer and Anna Willard)