The political center, eroded by nationalist forces aligned with President Trump, held in the end. But only just.

Presidential elections on Sunday in Romania and Poland — the two most populous countries on Europe’s formerly communist eastern fringe — halted, or at least slowed, a hard-right breakthrough that many liberals had feared.

But they also pointed to growing discontent with established political parties, no matter their ideological tilt, a trend that is likely to generate future turbulence as old and predictable loyalties fade.

In Romania, a centrist mayor who campaigned as an independent untainted by close ties to two long-dominant mainstream parties — widely viewed as corrupt — defeated another outsider, a hard-line nationalist who aligned himself with Mr. Trump and had been seen as the front-runner.

Nicusor Dan, the mayor of Romania’s capital, Bucharest, won a decisive victory over George Simion in a runoff for Romania’s presidency, confounding expectations of a sharp turn to the right. He won 54 percent of the vote, boosted by an unusually high turnout of 64 percent. That turnout was nearly 10 percent more than in the first round, in which Mr. Simeon trounced Mr. Dan and nine other candidates.

The result Sunday delighted mainstream political leaders across Europe and also the European Union’s executive arm in Brussels, whose president, Ursula von der Leyen, congratulated voters for having “chosen the promise of an open, prosperous Romania in a strong Europe.”