ROME-—Italians voted to shrink the size of the national parliament, according to exit polls, in an expression of Europeans’ continuing frustration with their political class.

More than 60% of voters approved a constitutional amendment that cuts the number of lawmakers by about one third, exit polls taken after a referendum held on Sunday and Monday showed.

The change is a victory for Italy’s 5 Star Movement, an eclectic antiestablishment party that has channeled popular anger with a political caste that many Italians see as privileged and out of touch with ordinary people. The referendum shows the continuing strength of populist sentiment in Europe that blames the shortcomings of political elites for countries’ economic and other woes.

Europe’s political establishment has this year hoped that the new challenges of the coronavirus pandemic would lead voters to prefer the governing experience of traditional parties, dampening fervor for political insurgencies from the nativist right or the anticapitalist left. But both the referendum and Italian regional elections, held at the same time, suggested the pandemic hasn’t weakened voters’ appetite for punishing longtime incumbents.

“Populism is far from dead. The antiestablishment sentiment is still pronounced in European societies,” said Catherine De Vries, a political scientist at Milan’s Bocconi University.

The pandemic has boosted the approval ratings of some national leaders, including Italian Prime Minister Giuseppe Conte and German Chancellor Angela Merkel. But that doesn’t necessarily translate into more votes for the parties that underpin them.

Italy’s governing center-left Democratic Party, a pillar of the country’s political establishment, faced a strong challenge in Monday’s regional elections from the nationalist opposition League led by Matteo Salvini, known for his anti-immigration rhetoric. Exit polls suggested the Democrats would defeat the League only narrowly in Tuscany, a region that has been solidly left-wing for decades.

Paradoxically, the Democrats supported the constitutional change to shrink the size of parliament, despite misgivings. Mr. Conte’s government is an awkward coalition of the Democrats and the 5 Star Movement, who put aside their longstanding animosity and formed a pact last year mainly to prevent Mr. Salvini from winning snap national elections.

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The Democrats reluctantly backed the 5 Star’s demand to shrink Italy’s Chamber of Deputies, the lower house of parliament, to 400 elected members from 630, and the Senate, or upper house, to 200 from 315. The change will take effect when Italy next holds parliamentary elections, which are due by 2023 at the latest but could come earlier if the governing coalition breaks up.

The 5 Star has long campaigned to cut back the number of politicians, attacking parliament as a gravy train that offers high salaries and expense accounts, sumptuous meals and free chauffeured limousines. In recent years, pressure from 5 Star lawmakers has forced parliament to cut back on some privileges for its members, including subsidized offices and costly, no-bid contracts for parliamentary supplies.

Parliament rejected a proposal to cut members’ salary in half, however. Italian lawmakers earn about €10,000 a month, 50% higher than their British peers.

Write to Giovanni Legorano at