BRUSSELS — After long indulging him, leaders in the European Union now widely consider one of their own, Prime Minister Viktor Orban of Hungary, an existential threat to a bloc that holds itself up as a model of human rights and the rule of law.

Mr. Orban has spent the past decade steadily building his “illiberal state,” as he proudly calls Hungary, with the help of lavish E.U. funding. Even as his project widened fissures in the bloc, his fellow national leaders mostly looked the other way, committed to staying out of one another’s affairs.

But now Mr. Orban’s defiance and intransigence has had an important, if unintended, effect: serving as a catalyst for an often-sluggish European Union system to act to safeguard the democratic principles that are the foundation of the bloc.

Early this year, the European Court of Justice will issue a landmark decision on whether the union has the authority to make its funds to member states conditional on meeting the bloc’s core values. Doing so would allow Brussels to deny billions of euros to countries that violate those values.

a new media law that curbed press freedom. It overhauled the country’s justice system, removed the head of its Supreme Court and created an office to oversee the courts led by the wife of a prominent member of the governing party, Fidesz. Election laws were changed to favor the party.

External factors strengthened Mr. Orban as well, including in 2015 when a record number of migrants made their way to Europe and when the right-wing Law and Justice party of Jaroslaw Kaczynski came to power in Poland. He suddenly had an ally there, and his tough stance against migrants won him support elsewhere, too.

Mr. Orban quit the conservative alliance when it became clear that it was going to oust his party.

Mr. Weber still regrets the loss of Fidesz. “On one level, it is a relief,” he said. “But Orban leaving is not a victory, but a defeat” in the effort to hold the center-right together as “a broad people’s party.”

It has helped Mr. Orban that the European Union has few and ineffective instruments for punishing a backsliding nation. Even the Lisbon Treaty, which gave enhanced powers to the European Parliament, has essentially one unusable tool: Article 7, which can remove a country’s voting rights, but only if passed by unanimity.

according to studies by R. Daniel Kelemen of Rutgers University and Tommaso Pavone of the University of Oslo, the commission sharply reduced infringement cases after the addition of new member states in 2004. José Manuel Barroso, a former commission president, “bought into this to work more cooperatively with governments and not just sue them,” Mr. Kelemen said. Mr. Barroso declined to comment.

Attitudes have shifted. With taxpayer money at stake, the next seven-year budget in the balance and the disregard for shared values shown by Mr. Orban and Mr. Kaczynski on leaders’ minds, Brussels may have finally found a useful tool to affect domestic politics, with a mix of lawsuits charging infringement of European treaties combined with severe financial consequences.

A marker has finally been laid down, Mr. Reynders said.

The big moment comes this month, when the European Court of Justice issues its ruling.

If Hungary and Poland lose the case, as expected, it is unclear what will happen if both countries simply refuse to comply. The European Union will be thrust deeper into unknown territory.