ROME — The U.S. ambassador to Italy was at the center of a political firestorm Wednesday after warning that the defeat of an upcoming Italian referendum on constitutional changes would hurt Italy’s chances of attracting American investment.

The vote has become a make-or-break judgment on the government of Premier Matteo Renzi, who has said the constitutional changes are needed to end Italy’s chronic political instability. Renzi has staked his administration’s survival on passage, which polls suggest is not at all certain.

Lawmakers backing a “no” vote sharply criticized Ambassador John Phillips’ comments as U.S. interference in Italy’s internal affairs. Some called for his removal.

Even the Italian president, Sergio Mattarella, stepped into the fray Wednesday to insist on the sovereignty of Italian voters to decide the referendum’s outcome.

Phillips made the remarks at a panel discussion at the Rome-based Center for American Studies on Tuesday. He told Italy’s foreign minister and other assembled guests that political stability was crucial to securing American investment.

While saying the choice belonged to Italian voters, Phillips said the United States believed the proposed changes “offer real hope and real opportunity for stability of government going ahead.

“All I can say from the American perspective is if it fails, those American companies that are coming to Italy — and Italy ranks right now eighth in investment in EU where it should be third or even second — it will be a big step backward in terms of attracting more foreign investment.”

The U.S. Embassy said the ambassador’s comments were in line with U.S. support of Renzi’s reforms.

The referendum remarks marked the second time Phillips has sparked controversy this year. He raised eyebrows in March by telling an Italian newspaper that Italy had committed up to 5,000 troops for an international force to help stabilize Libya.

At the time, the comments suggested that military intervention was near. The U.S. Embassy downplayed them and insisted that the figure had been floated previously by Italian officials.

Renzi’s referendum, scheduled for later this year, would greatly reduce the power of the Senate, abolish the provinces and give the central government control of some policy areas that now lie with the regions.

Renzi has said he would resign if the referendum fails, further motivating opposition forces to rally against the measure try to take advantage of widespread popular anger at slow economic growth, corruption scandals and the government’s performance.

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