ROME (Reuters) - Donald Trump’s unexpected victory in the U.S. presidential election is likely to make it even harder for Italian Prime Minister Matteo Renzi to win a crucial referendum on constitutional reform set for December.

Italy's Prime Minister Matteo Renzi talks to the media as he leaves a European Union leaders summit in Brussels, Belgium, October 21, 2016. REUTERS/Francois Lenoir

Renzi was one of the few world leaders to publicly endorse Hillary Clinton and Trump’s triumph has not only put the Italian premier in a difficult diplomatic position, it has also underlined the rise of anti-elite sentiment around the world.

“This factor is weighing against Renzi because at the moment for many Italians he represents the establishment,” said Roberto D’Alimonte, politics professor at Luiss University in Rome.

Renzi has said he will resign if he loses next month’s ballot and was already facing fierce headwinds, with all opposition parties pitted against him and almost every opinion poll over the past two months showing the ‘No’ camp ahead.

However, a large number of Italians remain undecided, helping to fuel government confidence that the ‘Yes’ vote will eventually win and stave off a renewed bout of political uncertainty that is feared by the financial markets.

“The Trump win does show that populism continues in 2016 and suggests a ‘no’ vote in the Italian referendum could be stronger than we assumed,” David Zahn, the head of European fixed income at U.S. fund manager Franklin Templeton, told Reuters.

The spread between Italian and German 10-year benchmark bonds was up 7.8 basis points at 1200 GMT at 154.05, the highest level since June when the closely watched gap jumped in the wake of Britain’s vote to leave the European Union. [IT10DE10=TWEB]

The constitutional reform proposes drastically curbing the role of the upper house Senate, a move that Renzi says will simplify decision-making and ensure stable government. Opponents say it will make the legislative process more complicated and reduce checks and balances.

“WE ARE THE BARBARIANS!”

Italy’s anti-establishment 5-Star Movement currently heads the opinion polls and its founder, Beppe Grillo, hailed Trump’s victory, seeing it as a vindication of his own maverick stance.

“It is those who dare, the obstinate, the barbarians who will take the world forward. We are the barbarians! The real idiots, populists and demagogues are the journalists and the establishment intellectuals,” Grillo wrote on his blog.

Jumping on the U.S. election bandwagon with an eye to the forthcoming referendum, former prime minister Silvio Berlusconi’s Forza Italia (Go Italy!) party put out Tweets saying “In America, they voted NO”.

The group’s parliamentary leader Renato Brunetta called on Renzi to resign immediately and not even wait until Dec. 4, saying his pro-Clinton stance had weakened Italy’s standing.

“From this day forward Matteo Renzi is politically finished, he is a dead man walking,” Brunetta said in a statement. “No other European country sided with one of the two contenders like Italy did. Now Renzi must reap the consequences and take responsibility for his bad choices.”

Renzi himself offered Trump his congratulations and said Italy’s relations with the United States, its NATO ally, were “solid”.

The new U.S. president is due to visit Italy next May when the country hosts the 2017 summit of Group of Seven leaders.