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The European Union faces a crisis that may prove its most existential threat yet. It has nothing to do with the opportunistic predations of Russia or the anti-Brussels exertions of Brexiteers. It’s not about the fiscal woes of the euro zone or a surge of migrants or the toll of a pandemic. It has to do, instead, with Poland.

Last week, Poland’s constitutional court, which is packed with judges loyal to the country’s illiberal ruling party, determined that the Polish constitution trumped European law in certain cases and superseded the authority of the European Union’s Court of Justice. Its judgment came amid a months-long political and legal tussle on the continent over what experts and many E.U. officials view as the right-wing Polish government’s steady assault on the country’s independent judiciary, a process that has played out in slow motion since the ruling Law and Justice party came to power in 2015.

The Polish court’s decision has mammoth implications. In an interview with Axios, E.U. Justice Commissioner Didier Reynders cast the situation as Europe’s Jan. 6 moment, a brazen assault on the legal and political consensus that unites the continental bloc. “During many years, we have had in our minds that it was granted that if you are a member of the EU, of course you apply the rule of law; you have full respect for democracy, fundamental rights and so on — maybe with some concerns but with a real intention to adapt your legislation to be in full compliance [with EU law],” Reynders said.

Poland now appears bent on going the other direction. At a testy plenary session Tuesday of the European Parliament in Strasbourg, France, Polish Prime Minister Mateusz Morawiecki squared off against European Commission President Ursula von der Leyen.

The latter declared in a speech that the Polish constitutional court’s “ruling calls into question the foundations of the European Union. It is a direct challenge to the unity of the European legal order.” She laid out possible avenues to sanction Poland, including the suspension of some of Poland’s rights and the use of various E.U. legal instruments to withhold dedicated funds to Warsaw. Morawiecki fired back at von der Leyen, casting his country as a victim of an “attack” from Brussels. “If you want to make Europe into a nationless superstate, first gain the consent of all European countries and societies for this,” he said.

But most observers outside of Poland view it the other way — that Warsaw’s gambit is a grievous blow to the European project. “The whole functioning of the E.U. is based on it being a rule-of-law political system,” R. Daniel Kelemen, professor of political science and law at Rutgers University, told Today’s WorldView. “The E.U. is not a state. … What it does have, ultimately, is a big body of law, treaties and regulations it has adopted and a commitment by member of states” to abide by them.

Once that edifice starts to erode, a precipitous unraveling could follow. “If courts across the EU cannot trust their Polish peers, then the EU’s legal system starts to gum up,” observed the Economist. “An arrest warrant here is not honored there; a banking license granted in one country may not be honored in another. Over time, an area over which people, goods, capital and services can flow freely turns into one where they can move only with trouble.”

The focus, now, shifts to how the bloc as a whole will deal with Poland’s rogue behavior. Its anti-LGBT laws already elicited censure. Now, there’s enthusiasm in some capitals to take a hard-line approach, including using a rule-of-law mechanism to block E.U. funds and pandemic relief to Warsaw without having to win unanimity among the 27-member state bloc.

The threat of such sanctions have incensed Polish officials. Ryszard Terlecki, the ruling party’s deputy leader, gestured last month to the notion that his country could follow in Britain’s footsteps. “The British showed that the dictatorship of the Brussels bureaucracy did not suit them and turned around and left,” he said.

But the prospect of a “Polexit” remains distant, not least because an overwhelming majority of Poles want to remain in the European Union. Advocates of tougher action on the E.U.'s part think it’s time to draw a line in the sand. “If the commission and a majority of the member states do not take a stand, other countries, such as Bulgaria, Hungary, and Slovenia, will exploit these weaknesses to prevent Europe from pursuing the path to more integration,” wrote Carnegie Europe senior fellow Judy Dempsey. “For these countries, more integration is seen as undermining national sovereignty. But that is what all EU members signed up for when they joined the bloc.”

The way forward looks murky. For years, figures like von der Leyen and her former boss, German Chancellor Angela Merkel, accommodated illiberal governments in countries like Poland and Hungary both on partisan grounds — Hungary’s ruling Fidesz until recently was part of the same European political bloc as Merkel’s Christian Democrats — as well as a recognition of the complexity in taking on a government of a member state.

“The legal remedies available to the Commission, including a new, yet-to-be-triggered enforcement mechanism that could restrict the disbursal of EU budget funds, are insufficient — limited, time-consuming, cumbersome, impossible to carry out, or all of the above,” Politico noted. “But the political reality is that the EU cannot afford to go to war with one of its own member countries without putting its entire agenda in danger of being blocked, given that all crucial policy decisions require unanimity.”

The dilemma underscores a key tension underlying how the E.U. works. Brussels has the capacity to penalize and fine huge multinational tech companies like Amazon and Google, “yet it seems brought to its knees by an economically dependent member state,” Kelemen said.

Kelemen argued that Brussels needs to recognize and use its real leverage. “The gamble you’re getting from these regimes” — that is, Poland and Hungary under democracy-eroding Prime Minister Viktor Orban — “is that the E.U. will wimp out and kick the can down the road,” he said. “They want to make the E.U. a hospitable home for them to have an electoral autocracy that can last for decades.”

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