BRUSSELS — As Prime Minister Viktor Orban steadily established an “illiberal state” in Hungary, dismantling the country’s checks and balances, stacking its constitutional court with loyalists and creating a template for other far-right leaders, a powerful group of European politicians took note.

And said little.

Mr. Orban is now seen as a threat to Europe’s mainstream leadership, especially the conservative alliance that for years chose to shelter him. Leaders of Europe’s conservative political parties — including Chancellor Angela Merkel of Germany — refrained from reining him in, largely because he was part of their coalition in Brussels, and because they thought they could control him.

Now some leaders in the alliance, known as the European People’s Party, have concluded that was a mistake, and are breaking from Mr. Orban. On Wednesday, the European Parliament voted to suspend Hungary’s voting rights within the European Union — an extreme measure whose outcome hinged on the willingness of conservatives to turn against the prime minister.

The is only the first step in a showdown with Mr. Orban. Punishing Hungary would also require a vote of the leaders of the European Union’s 28 member states, and approval is far from certain.