The warning will also be seen as a clear signal to several other former Communist countries, including the Czech Republic, Hungary, Romania and Slovakia, which are all led by parties that espouse populist policies and which have taken, or are considering, actions viewed by critics as threatening to democracy.
The dispute between Poland and other members of the European Union has been building for more than two years, since the populist Law and Justice party swept into power promising to rid Poland of corruption and the remnants of communism.
It is now coming to a head after the Polish legislature approved an overhaul of the judicial system that would force out about 40 percent of Supreme Court justices and would allow politicians to play a greater role in new appointments. The main opposition party, Civic Platform, has decried the proposals as paving the way to the end of democracy in Poland.
The Venice Commission, a legal group attached to the Council of Europe, an international rights organization, echoed that concern, saying that the rules put the integrity of the Polish judicial system at risk.
“It’s a very serious thing, the first time in the history of the E.U. that this procedure will be officially started,” said Vladimir Bartovic, director of the Europeum Institute for European Policy, a nonprofit research group based in Prague that focuses on Central and Eastern Europe.
“It’s eventually up to the decision of the Council of course, but already the fact that the commission dares to propose something so serious as the nuclear option, is already a huge issue,” Mr. Bartovic added.
President Andrzej Duda of Poland has not yet signed the changes into law. While not officially a member of the governing party, Mr. Duda has long been considered an ally of Jaroslaw Kaczynski, the leader of Law and Justice and the driving force behind the party’s rise to power.
However, that relationship was strained this summer when Mr. Duda vetoed similar legislation aimed at the judiciary. That proposal called for dismissing the entire Supreme Court and drew huge protests.
The move by the European Commission is not without risks. It could embolden Mr. Kaczynski and draw more support for his cause if people believe Poland is being unfairly targeted by what many view as distant elites in Brussels.
The Polish minister of foreign affairs, Witold Waszczykowski, expressed anger even before the commission’s vote, saying the decision was “an attempt to stigmatize Poland and push us aside when key decisions are made in the E.U.”
Before the European Commission’s vote on Wednesday, Joanna Kopcinska, a spokeswoman for the Polish government, told the state broadcaster that Warsaw rejected the criticism from Brussels.
“Obviously, we don’t ignore the message that comes from the European Commission about the threat to the rule of law in our country, but it’s absolutely unfounded,” she said. “The bloc has too many immediate problems that need to be solved to pay so much attention to Poland. Poland is a democratic and sovereign state, and there is nothing bad going on here.”
“Scaring Polish society with sanctions is unjustified,” she added.
Jacek Rostowski, a senior member of Civic Platform and deputy prime minister in the previous government, wrote on Twitter on Wednesday that sanctions would be an attempt by Europe to defend Poland’s rights and freedoms from the “creeping dictatorship” of the Law and Justice party.