Austrian Chancellor Sebastian Kurz speaks during a news conference in Vienna on May 22. (Christian Bruna/EPA-EFE/Shutterstock)

ELECTIONS TO the European Parliament usually don’t attract much attention, even from Europeans, but this week’s vote for the transnational legislature has defenders of liberal democracy around the continent worried. Far-right parties, which mix economic populism, hostility toward immigrants and authoritarian leanings with disdain for the European Union itself, have banded together across national borders and expect a surge of support. While no one expects the rightists to capture a majority in the polls across 28 E.U. members, they could gain enough seats to obstruct the Parliament and further weaken a union already stressed by economic stagnation and Britain’s pending departure.

The populists, who have the sympathy of President Trump, argue they are standing up for Europeans who want to defend their borders against waves of asylum seekers from the Middle East and Africa, and who are weary of the rule of an isolated E.U. elite in Brussels. But there is a darker side to the movement that includes tolerance of racist followers, alliance with the Russian regime of Vladi­mir Putin and an openness to corruption. The eruption of a new scandal involving Austria’s extremist Freedom Party vividly illustrates the problem.

Last week, German media reported on video footage showing the leader of the Freedom Party, Heinz-Christian Strache, partying with a woman who represented herself as the niece of a Russian oligarch and who offered to support the Freedom Party and purchase a stake in Austria’s largest tabloid newspaper. Mr. Strache promised to steer government construction contracts to her in return. The encounter seems to have been a sting operation — the woman was not an oligarch’s niece — and it’s not clear who made the video, or why. Mr. Strache nevertheless was forced to resign, and Austria’s coalition government, led by center-right Chancellor Sebastian Kurz, collapsed.

Mr. Kurz, a 32-year-old political wunderkind, embarked on a risky experiment when he chose to partner with the far right following a 2017 election, rather than forge a centrist alliance that isolated the extremists. The gambit backfired even before the emergence of the video. Having gained control of the powerful interior ministry, the Freedom Party cracked down on the anti-extremism unit of the domestic intelligence service. Meanwhile, Mr. Kurz was repeatedly forced to react to racist statements and ads by his coalition partners. “The Freedom Party,” he said Saturday, “has damaged the country’s image.”

The drama should offer a lesson to politicians in other countries, such as Italy, Denmark and Estonia, who have allied with the far right. In Austria, voters appear to have taken heed: Initial polls show a sharp drop in support for the Freedom Party. But as the elections across Europe this week are likely to show, the broader wave of right-wing populism has yet to crest.