A tight presidential election this weekend in Poland hinges heavily on young voters, a restive demographic with a big stake in the direction of the country.

But the two candidates facing off in a runoff election on Sunday share a big problem: Young Poles do not much like either of them.

That is adding to the unpredictability of an election widely seen as pivotal for the future of the biggest economic and military power in the European Union’s formerly communist East. It is also critical to efforts by the country’s centrist prime minister, Donald Tusk, to unravel the legacy of his populist predecessor.

In a first round of voting on May 18, voters aged 18 to 29 overwhelmingly supported anti-establishment candidates who failed to make it to the runoff. They mostly shunned the candidates competing on Sunday, who represent Poland’s two dominant political parties — Civic Platform, led by Mr. Tusk; and Law and Justice, the former governing party led by Jaroslaw Kaczynski.

The runoff pits Rafal Trzaskowski, the liberal mayor of Warsaw who is backed by Mr. Tusk’s party, against Karol Nawrocki, a nationalist historian and former boxer supported by Law and Justice.