Brussels and Warsaw brought their years-long dispute to the European Parliament on Tuesday in a tense session in front of MEPs.
Their relationship was plunged into crisis earlier this month when a Polish court ruled some of Poland's laws had primacy of EU ones.
"We cannot and we will not allow our common values to be put at risk," said Ursula von der Leyen, president of the European Commission.
But Polish Prime Minister Mateusz Morawiecki, also present at the plenary session in Strasbourg, hit back.
"Poland will not be intimated," said Morawiecki, vowing to defend his country's sovereign independence against the "creeping expansion" of EU institutions.
The battle of words illustrated the poor state of relations between the two sides, which this month reached a new low after the Polish government endorsed a defiant judgment of the country's constitutional court that questioned EU law primacy.
The judges, in a majority decision, objected to the transfer of sovereignty from Poland to the EU and rejected the supreme legal authority of the EU's Court of Justice (ECJ), based in Luxembourg.
The ruling is seen as a "nuclear strike" on the primacy of the EU law, one of the union's cornerstone principles. Established back in 1964 as a result of the precedent-setting Costa v ENEL case, the principle states that, in all cases where the EU has competence, the bloc's laws have priority over national legislation, even over a country's constitution.
Von der Leyen used her speech to defend the principle.
"This ruling calls into question the foundations of the European Union. It is a direct challenge to the unity of the European legal order. Only a common legal order provides equal rights, legal certainty, mutual trust between member states and therefore common policies," she said.
"This has serious consequences for the Polish people. Because the ruling has a direct impact on the protection of the judiciary," she added. "Without independent courts, people have less protection and consequently their rights are at stake."
Von der Leyen reminded MEPs that the ruling followed years of threats to the independence of the judiciary in Poland, including the creation of a controversial disciplinary chamber to punish judges and remove them from office.
Both the European Commission and the EU's Court of Justice have found these reforms to be in breach of EU law and requested their immediate dismantling, but Warsaw has refused to abide by the Luxembourg verdicts.
Morawiecki personally filled a petition before Poland's highest-ranking tribunal to know if the ECJ was exceeding its competencies. This dispute led to the recent verdict.
"When joining the EU the Polish people put their trust in the European Union. They expected the EU to defend their rights. And rightly so," von der Leyen told the European Parliament.
"The Commission is the guardian of the Treaty. It is my Commission's duty to protect the rights of EU citizens, wherever they live in our Union. The rule of law is the glue that binds our Union together."
Von der Leyen listed the three options her team has at its disposal to reassert EU law primacy inside Poland: opening new infringement procedures, triggering Article 7 to deprive Poland of its voting rights, and activating the conditionality mechanism that links EU funds with respect for EU values. The first two options have already been tried and tested, with questionable results.
All eyes are now on the new budgetary mechanism, which could freeze payments for Poland, the largest recipient of EU funds. The scheme, applicable since January, is based on a complex procedure and exchange of notifications between Brussels and the government suspected of breaching EU law. It could take up to nine months for the European Council to approve the final suspension of payments.
"In the coming years we will be investing €2 trillion with the Multiannual Budget and the NextGenerationEU recovery programme," von der Leyen said.
"This is European taxpayers´ money. And if our Union is investing more than ever to advance our collective recovery, we must protect the Union budget against breaches of the rule of law."
The European Commission president closed her speech calling for dialogue to resolve the situation and praising Poland for its contribution to European integration.
"Europe has benefited from Poland's unique experience. Without the people of Poland, our European journey would have been very different."
'Poland will not be intimated'
In a lengthy and far-reaching speech, Morawiecki defended the contentious ruling and promised to uphold his country's sovereign rights against what he described as a gradual and unlawful encroachment of competencies by EU institutions.
Morawiecki began his rebuttal listing a series of unrelated challenges that Europe is currently facing, such as soaring electricity prices, illegal migration, rising public debt, tax havens and the increasingly aggressive behaviour of the Russian Federation. The premier particularly insisted on the dangers of the ongoing energy crunch, warning the crisis could result in "millions" of bankrupt businesses and citizens pushed into poverty.
He then reflected upon Poland's EU membership and said his government, despite the present difficulties, was committed to advancing the "common project".
"For us, European integration is a civilisational and strategic choice," he said. "We are here, we belong here and we are not going anywhere."
Morawiecki pointed out that western countries, and especially French and Germany, have benefited greatly from the entrance of eastern states into the bloc. But, he added, this west-east divide has resulted in first- and second-class member states that are treated differently.
"Today all Europeans, expect one thing. They want us to face up to the challenges posed by several crises at the same time, and not against each other, looking for someone to blame - or rather, those who are not really to blame, but whom it is convenient to blame," he said.
"We cannot remain silent when our country - including in this Chamber - is attacked in an unfair and biased manner."
Morawiecki said Poland was a "proud country" and that threats of financial penalties – under the proposed conditionality mechanism – amounted to "blackmail".
"I reject the language of threats, hazing and coercion."
The PM then tackled what was perhaps the central point of his argument: the Polish constitution is the country's highest legal act and therefore must be protected against the "unacceptable" expansion of EU competencies. In his view, the European Union has exceeded the powers granted by member states in the original treaties and countries should be entitled to react to this abuse.
Morawiecki, however, didn't specify what competencies the EU was exceeding. The treaties list a series of areas where the EU has exclusive or shared competence and empower both the Commission and the Court of Justice to take action when a member state fails to fulfil an obligation under EU law.
"The Union will not fall apart from the fact that our legal systems are different." he said, depicting the ECJ as leading a "silent revolution" through its verdicts.
Last month, the Commission asked the Luxembourg tribunal to impose daily fines on Poland after Warsaw failed to dismantle the disciplinary chamber of judges.
Towards the end of his speech, Morawiecki was interrupted by the chair of the European Parliament for overstepping his allotted speaking time.
The Polish PM said he had been given 35 minutes to speak and continued reading his statement, with various warnings against a European "nationless superstate".
"We say yes to European universalism and no to European centralism" he said. "Poland will not be intimidated".
Morawiecki said his government was willing to engage in dialogue with Brussels "based on checks and balances" and proposed the creation of a judicial chamber comprising judges from the ECJ and national constitutional courts in order to resolve legal conflicts.
"Long live Europe, the greatest place in the world," he concluded.
It's still unclear what next steps Brussels or Warsaw might take to deescalate tensions. The Commission is reportedly preparing the activation of the conditionality mechanism, although given its protracted estimated timeline, the scheme could be better used as a symbolic threat to encourage dialogue and find a solution rather than as an instrument to impose financial sanctions.
"I don't think dialogue or serious normal dialogue is now timely because the Polish government, especially the more extreme parts of the Polish government, are not interested in dialogue. They're interested in hijacking the Polish judiciary system," Marek Belka, a Polish MEP from the S&D group, told Euronews after the plenary.
"The European Commission goes into politics beyond the treaties, this is why we protest," said Witold Jan Waszczykowski, a Polish MEP from the European Conservatives and Reformists (ECR) group.