ROME (AP) — Italians voted Sunday in a referendum on constitutional reforms that Premier Matteo Renzi has staked his political future on, hoping to survive the rising populist forces that have gained traction across Europe.

Renzi has said he will resign if the reforms, which he contends will modernize Italy and reduce its legendary bureaucracy, are rejected. Opposition politicians, ranging from the far-right to the far-left have vowed to press for a new government if voters reject Parliamentary legislation overhauling much of the post-war Constitution.

Even some figures in Renzi's Democratic Party, including ex-Communists, said they'd vote against the reforms.

The premier made no comment as he voted Sunday in Pontassieve, a Tuscan town east of Florence, along with his wife, Agnese Landini. He was to return to Rome to watch the outcome of the vote.

Some of Renzi's political opponents were hoping to tap into the populist sentiment that has been gaining ground with the U.K. vote in June to leave the European Union and the U.S. presidential victory last month by billionaire political outsider Donald Trump.

A "yes" vote would strengthen Renzi's 2 ½-year-old government, giving it impetus to complete its five-year term and time to prepare for elections in 2018, while a "no" vote would favor early elections sometime next year.

More than 46 million Italians were eligible to cast votes while another 4 million were registered to vote abroad. The overseas votes were being tallied under guard at a warehouse outside of Rome.

The risk of political instability in Italy, Europe's fourth-largest economy, triggered market reaction before the vote, with bank stocks sinking and borrowing costs on sovereign debt rising.

Italy's European Union partners were closely monitoring the vote Sunday, which fell on the same day as a presidential runoff in Austria, where left-leaning candidate Alexander Van der Bellen won against a right-wing populist to cheers from mainstream politicians.

The referendum in Italy aims to streamline Italy's cumbersome lawmaking process by reducing the powers of the Senate, while also removing some key decision-making powers from regions.

Renzi has argued the reforms will reduce administrative wrangling that often winds up in the courts. He says that will make Italy more attractive to investors and will help him relaunch the country's moribund economy. But his pledge to tie the outcome to his political future has turned the vote into a plebiscite on his leadership.

A headline in the Frankfurter Allgemeine Zeitung said Renzi's "arrogance is his shortcoming," noting that "Europe is at stake" in the vote. The Guardian newspaper noted that the referendum was among a series of votes in Europe that could "conceivably herald the end of the European project in its current form."

The chief rival of Renzi's Democratic Party is the populist 5-Star Movement, whose co-founder, comic Beppe Grillo, blames the common euro currency and European Union rules for much of Italy's woes.

Spearheading the "no" campaign, Grillo voted Sunday evening in Genoa.

"We did a great job, thus the important thing is, if we must lose, we don't blame anybody. If the Italians choose something else, we'll respect it," Grillo said.

Grillo has cautioned supporters to be on guard for any vote tampering, especially with ballots cast by Italians abroad, who are generally expected to favor the reforms.

Three former premiers — Silvio Berlusconi, Massimo D'Alema and Mario Monti — have come out against the reforms for different reasons. Berlusconi has argued that it concentrates too much power in the premier, while Monti says the reforms don't go far enough.

How the vote plays out politically is likely to depend on the turnout and the margin of the decision.

By 7 p.m., four hours before the polls close time, some 57.2 percent of eligible voters had cast ballots, according to Interior Ministry figures, beating previous turnouts for constitutional referendums in 2006 and 2011.

If voters reject the referendum, Renzi was expected to offer his resignation right away. But analysts say President Sergio Mattarella, whose job it is to designate someone to form a new government, is unlikely to act on such an offer until parliament passes a new budget law.

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Barry reported from Milan.