For the past decade, populists like Mr. Babis have often seemed politically invincible, rising to power across Central and Eastern Europe as part of a global trend of strongman leaders disdainful of democratic norms. But on Saturday, the seemingly unbeatable Mr. Babis was defeated because opposition parties put ideological differences aside and joined together to drive out a leader they fear has eroded the country’s democracy.

Their success could have major repercussions in the region and beyond. In Hungary and in Poland, where nationalist leaders have damaged democratic institutions and sought to undermine the European Union, opposition leaders are mobilizing, trying to forge unified fronts and oust populist leaders in upcoming elections.

“Populism is beatable,” said Otto Eibl, the head of the political science department at Masaryk University in Brno, the South Moravian capital. “The first step in beating a populist leader is to suppress individual egos and to compromise in the interest of bringing a change.”

The biggest showdown could come in Hungary, where Prime Minister Viktor Orban has promoted himself as Europe’s standard-bearer for “illiberal democracy,” while his Fidesz party has steadily stripped away democratic checks, squeezing independent media and the judiciary.