REGGELLO, Italy — The stage was set for war. Literally. Inside a small Tuscan theater with a mock-up of a World War I trench, Susanna Ceccardi, a rising star of Italy’s hard-right League party, was flanked by rival candidates for the European Parliament elections and firing angry salvos against a club she soon hopes to join.

“This Europe must be changed, this Europe of bureaucrats, do-gooders, bankers, boats of migrants, it has to be changed,” Ms. Ceccardi, the 32-year-old mayor of Cascina, Italy, roared to smatters of applause.

She is among scores of nationalist candidates from across the Continent who are vying to win an office at the heart of the European Union — so they can break it from the inside.

[See the results of the European Union elections.]

Not so long ago, Europe’s populist movements were advocating a departure from the bloc, or at least from the euro currency area. But with voters overwhelmingly in favor of staying in — an attitude hardened by two years of Brexit chaos — that strategy has changed: Now they are promising an insurgency from within.