By Crispian Balmer

ROME (Reuters) - Italian Prime Minister Matteo Renzi heads to the United States on Monday for a high-profile visit that he hopes will boost his flagging campaign for a referendum on constitutional reform.

He will be guest of honor on Tuesday at what looks certain to be U.S. President Barack Obama's farewell state dinner, a star-studded affair that will serve as a stamp of approval from Washington for Renzi's reform efforts.

"We didn't pick the date for the dinner, but it comes at a very welcome time and clearly shows that Renzi has the support of the world's most powerful leader," said a senior Italian government official, who declined to be named.

Italians will vote on Dec. 4 on the constitutional overhaul, which the center-left prime minister says will bring greater political stability to the country by curbing the powers of the upper house Senate and streamlining the lawmaking process.

Renzi has said in the past that he will resign if he loses the ballot, but in recent weeks he has declined to repeat the pledge as a slew of opinion polls have suggested he will not prevail.

The U.S. ambassador in Rome has already spoken out in favor of the reform proposals, drawing the ire of opponents who denounced his comments as unwanted foreign interference. Obama might not speak out so openly, but his support will be clear.

"I think the message Obama wants to send is that he has a lot of respect for Renzi. This cannot hurt the prime minister's standing at home ahead of the referendum," said Franco Pavoncello, the president of John Cabot University in Rome.


Renzi, 41, has worked hard to win White House respect.

He has been one of the most vocal supporters in Europe of free trade deals championed by Obama, but viewed with great scepticism by the center-left in Italy.

Renzi has also risked the wrath of his political family by allowing the U.S. military to use Italian bases for strikes in Libya, and only last week Rome announced it was joining a new NATO force in eastern Europe that has angered Russia and is viewed with suspicion on Italy's left.

In addition, he was the first European leader to openly back U.S. Democratic presidential candidate Hillary Clinton in the Nov. 8 election, alarming some of his advisers who fear a backlash should Donald Trump triumph.

The last time the United States threw a state dinner for Italy was in 1998. To mark the occasion this time, Italian-American celebrity chef Mario Batali will prepare the food while Grammy-winning Gwen Stefani will sing for the 500 guests.

Among those present will be fashion designer Giorgio Armani; the Italian chef of Europe's CERN research center, Fabiola Gianotti; and Giusy Nicolini, the mayor of Lampedusa - a tiny Italian island that has taken in thousands of migrants.

"Tomorrow, our country will be the guest for President Obama's final state dinner. This is a symbolic fact, not a personal question. It is the recognition from our dear, main ally of how strong our friendship is," Renzi said on Monday.

Another star attendee on Tuesday evening will be Oscar-winning actor Roberto Benigni, who last week announced that he would be voting "yes" in December.

His invitation was condemned by the "No" camp, which saw it as an electoral maneuver. "A dinner award for 'Yes' at the referendum. How squalid," tweeted Renato Brunetta, parliamentary leader of Silvio Berlusconi's center-right Forza Italia party.

The pro-American Berlusconi was never awarded a state dinner in Washington during his nine years as prime minister.

(Editing by Mark Heinrich)