EU Council President Donald Tusk holds up a copy of the Polish constitution as he speaks to supporters at Warsaw University | Janek Skarzynski/AFP via Getty Images Tusk calls on Polish government to obey constitution The European Council president defends his foray into domestic politics.

WARSAW — Donald Tusk, president of the European Council, dove back into national politics on Friday, giving a speech in the Polish capital that admonished the ruling Law and Justice (PiS) party for not obeying the country's constitution.

Speaking on Poland's national day commemorating the country's first constitution from 1791, Tusk said: "It can't be that those in power celebrate the constitution once a year and then disrespect it on a daily basis."

Poland's government, in power since 2015, has tangled with the EU over changes made to the country's judicial system, which it says are needed to root out the vestiges of communism. Critics see it as an attempt to bring the courts under tighter political control, and both the EU and the Council of Europe have warned that many of the government's measures violate Poland's constitution.

Tusk hammered home the point during his speech at the University of Warsaw by holding up a booklet of the Polish constitution and then reading from its preamble.

The former Polish prime minister has been flirting with a return to national politics when his term as Council president expires on December 1; Poland holds presidential elections next year and opinion polls show him neck-and-neck with the incumbent Andrzej Duda.

However, Tusk hasn't yet made a formal announcement of his intentions.

He swatted away criticism from pro-government media that his speech amounted to interference in the internal politics of a member state.

"Those who say that as the head of the European Council I should not get involved in the political campaign of a given party are right, as it's not the role of my office," Tusk said. "But it is my right and obligation as the head of the European Council to support Europeans in every EU country who are determined, often in difficult circumstances, to unite and not divide people."

The PiS government sees Tusk as a foe and a bitter rival. It tried unsuccessfully to block him two years ago from a second term as Council president. The ruling party's opponents see Tusk as the opposition's best chance of unseating PiS.

The opposition, led by Tusk's old party, Civic Platform, accuses PiS of working toward pulling Poland out of the EU in what it calls a "Polexit." The government vigorously rejects that charge, and during the ongoing campaign for the European Parliament election, PiS candidates have taken a much stronger pro-EU line than in the past.

Duda is even calling for Poland's membership in NATO and in the EU to be written into the country's constitution.

Tusk said he isn't opposed to Duda's idea, cuttingly calling the president "a former Euroskeptic who is now a Euroenthusiast."

But then he added: "Why amend the constitution if you're not prepared, for a variety of reasons, to obey its fundamental rules?"