ROME — Matteo Salvini basked in triumph on Monday after a thumping victory for his far-right League Party in this weekend’s European Parliament elections rendered him the dominant politician in Italy and the strongest claimant to the leadership of Europe’s populists.

But if Europe has been an incubator for resurgent nationalism in recent years, it now also feels like an active battleground.

With Europe’s decades-old project of unity increasingly in the balance, the voting energized both sides on a polarized Continent. It was a contest between angry, disaffected nationalists who want to beat back what they see as a remote and overreaching bureaucracy in Brussels, against the once-sleepy, complacent supporters of Europe looking to defend a unity that can no longer be taken for granted.