Anti-establishment 5Star Movement party leader, Beppe Grillo with a portrait of Italian Prime Minister Matteo Renzi | Tiziana Fabi/AFP via Getty Images 5 things Italians vote on this week The referendum on December 4 will be about more than constitutional reform.

FLORENCE, Italy — When they go to the polls on December 4, Italians will be voting on more than a constitutional reform.

The contest has effectively become a choice between center-left Prime Minister Matteo Renzi, who favors the EU despite his recent criticism of Brussels, and Beppe Grillo, the comedian-turned-politician and leader of the anti-establishment, Euroskeptical 5Star movement.

Renzi has threatened to resign if the vote doesn't go his way, and the political tone has become decidedly dire, with some analysts saying that an anti-government vote could mean an end to much-needed reforms — or even an Italian exit from the euro.

The governor of Italy's central bank, Ignazio Visco, has tried to quell the worst fears, saying in a recent interview that “it is dangerous to over-emphasize the importance of the referendum.” Yet after Brexit, Donald Trump's victory in the U.S. elections and a plebiscite in the Netherlands on a free trade agreement with Ukraine that was rejected, the Italian vote is seen as a popular verdict on European governance.

The proposed reform is ambitious, affecting almost a third of the articles in the Constitution. The amendments were already approved by parliament but without the two-thirds majority needed to avoid the referendum. Italians will have to say yes or no to a single question that sums up the gist of the reform on trimming Senate powers, reducing the number of senators and strengthening central state powers.

Here are five things that Italians will decide with their vote:

1. Senate powers: In a proposal that its backers say will make the decision process more efficient, Italians are voting on whether to curtail the Senate's power. The number of senators will drop from 315 to 100 and only certain laws will need the approval of both chambers. It isn't clear if the Senate would be directly elected but the reform states it would comprise 74 members of regional assemblies and 21 mayors. The remaining five senators would be appointed by the President of the Republic.

Critics say boundaries between the two chambers aren't clearly drawn and that this could trigger more competition between the two institutions. Some say it would have been better to get rid of the Senate altogether.

2. Central state powers: Italians have to decide whether to reduce the power of the regions in favor of Rome — the latest initiative from Renzi to centralize power also includes school and labor reforms.

This is a fix to a previous reform. In 2001, voters agreed to give more power to the 20 Italian regions. But that initiative, which was engineered by the center-left, ultimately created turf wars between local and national government, and critics say that the fact that the new reform is effectively a fix to a previous reform, shows how poorly designed the laws are and have been.

3. Electoral law: Indirectly, Italians also will vote to activate a new electoral law intended to make government more stable by giving the winning majority extra seats which would help consolidate power and avoid having to form coalitions. This could benefit Grillo who has always said that his movement will never be part of a coalition. However, even if Renzi wins, the Constitutional court still has to give its opinion on the electoral law and it is expected that it will reject at least some parts of the reform.

4. Grillo, the unknown: The 5Stars are neck-and-neck with Renzi's Democrats in the polls, although their government skills are still untested at a national level. In an interview a few years ago, Grillo talked about his ideas to deal with Italy's staggering public debt which, at 132 percent of GDP, is the second-highest in the eurozone after Greece. “We want to renegotiate the billions of euros of interest a year that are eating us alive.” Luigi Di Maio, the 5Star's most prominent figure who is expected to be prime minister if it ever wins power, has repeatedly talked about calling a referendum on the single currency. Yet the party's real intentions are still unknown. Its program contains no reference to a referendum on the euro or a renegotiation of the debt.

5. Renzi's future: Since the very beginning of the referendum campaign, Renzi has made this a vote about his leadership. And the former mayor of Florence is different from his predecessors Enrico Letta and Romano Prodi in that he is not only prime minister but also party leader. Which means that, unless he loses the vote in spectacular fashion, he remains in control of the largest party in Parliament, even if he steps down as prime minister. His resignation wouldn't necessarily bring early elections. The president of the republic, Sergio Mattarella, has the last say on that.

Should he win the vote, however, many expect it to strengthen his position on the European stage. “It will make his leadership in the European Socialists' party even stronger,” says Emanuele Fiano, a supporter and lawmaker from his party.