The EU launched an inquiry in January into whether Poland's euroskeptic government had breached the EU's democratic standards, in particular its attempted reform of the country's constitutional court.
The Rule of Law Framework was adopted in 2014 and this is the first time it has been used. It could lead to sanctions such as a suspension of Poland's voting rights in the EU's executive.
The commission will discuss the issue afresh on Wednesday.
The commission has been trying to reduce tensions with Warsaw, but there is growing doubt in Brussels that a way out of the stalemate can be found.
First Vice President of the Commission Frans Timmermans traveled to Warsaw last week for talks with Szydlo. "I fully agree with the Polish prime minister when she says this is only a Polish problem and that we can only find a Polish solution," he told reporters. But few think the commission's rule-of-law probe will result in official sanctions against Poland.
EU fosters defiance
But the commission has subsequently stepped back from executing any of its threats. Following talks in Warsaw, the EU executive decided not to launch a formal procedure against the Polish government's court plans, which Brussels fears undermines judicial independence and the rule-of-law.
Deputy Foreign Minister Konrad Szymanski said last week that Warsaw was ready to give ground to end the crisis, but Kaczynski has become increasingly defiant and belligerant in tone.
"The procedure that is currently being used against us is a non-treaty procedure, a made-up one, and it can be challenged in the Court of Justice of the European Union at any moment," Law and Justice leader Jaroslaw Kaczynski said.
"If it gets fierce, we will do this," he told the "Do Rzeczy" weekly in an interview on Monday.
What's the fuss about?
Poland's government - elected in September - has effectively sidelined the country's constitutional court after increasing the size of majorities needed to make rulings and changing the order in which cases are heard.
The EU has said the changes undermine the court's independence.
Ramping up the rhetoric
"There are no grounds to start a procedure of penalising Poland," Foreign Minister Witold Waszczykowski told private broadcaster TVN24 on Monday, adding that he hoped the commission would stop the procedure.
EU members should try to solve most problems at the "lowest possible level" and not delegate them to Brussels, Waszczykowski said. The EU, he said, should not "try to become a superstate," he said. In an interview with "Die Welt," Waszczykowski said national parliaments should have a greater voice in EU matters, also praising British Prime Minister David Cameron for renegotiating the terms of the country's membership.
But Waszczykowski rejected the idea that Poland could follow Britain's example and consider leaving the EU. The foreign minister said the government would not move forward with a "Polexit" and that "no respectable political force would do so."
"It's not Poland which has a problem with the EC, it's the EC which has the problem," Prime Minister Beata Szydlo said.
EU fights back?
Viviane Reding - who as commission vice president in 2014 helped create the rule of law procedure - said the EU shouldn't let Poland's "authoritarian drift" continue. "In no way does this amount to interfering in Poland's internal matters," Reding, now an MEP, said. "Our common Treaty values, such as the respect for the Rule of Law, are indivisible: if one Member State disrespects those values, this regards all of us."
Leverage for Brussels could be the 14 billion euros Warsaw gets annually from the EU, the largest beneficiary of the EU budget.
A recent opinion poll published in the Polish media said almost 40 percent of Poles "could consider" leaving the EU.
