Poland's ruling Law and Justice (PiS) party has been increasingly at loggerheads with the EU and Tusk since coming to office in late 2015, although the acrimony between Tusk and PiS dates back many years. The PiS is locked in disputes with the bloc over immigration, logging of an ancient forest and putting courts and media under more government control, as reported by Reuters.
Tusk, Poland's former prime minister and the arch-rival of PiS leader Jaroslaw Kaczynski, won a second term in March as chairman of EU summit meetings – with Poland the only country to vote against his extension.
"Alarm! A vehement dispute with Ukraine, isolation in the European Union, departure from the rule of law and independent courts, attack on non-governmental sector and free media – PiS strategy or Kremlin's plan?" Tusk tweeted. "Too similar to rest easy."
Read alsoEU's Tusk calls for swift decision on Russian pipeline – media Tusk was referring to, among other things, the fact that Ukraine summoned the Polish ambassador in Kyiv on Saturday after Poland denied entry to a Ukrainian official in an escalation of a diplomatic spat over the two neighbors' troubled past.
Tusk did not provide details of what he described as the "Kremlin's plan". In May, Tusk urged Group of Seven leaders on to stick to their sanctions policy on Russia over the Ukraine crisis. Tusk also sided with member nations such as Poland and the Baltic states in their efforts to oppose a new pipeline connecting Russia and Germany.
The Polish government denies all charges from Brussels that it is undermining the rule of law or isolating Poland in Europe, saying it needs to overhaul Poland's ineffective legal system and stand up for Poland's interests in the EU.
A majority of EU lawmakers on Wednesday demanded punishment for the eurosceptic government in Poland, saying it was undermining the rule of law and promoting intolerance.
In a response to Tusk's comment, Prime Minister Beata Szydlo tweeted:
"@donaldtusk as @eucopresident has done nothing for Poland. Today, using his position to attack the Polish government, he is attacking Poland."
In March, Poland's defense minister accused Tusk of working with Russian President Vladimir Putin to harm Polish interests following the 2010 plane crash that killed President Lech Kaczynski – Jaroslaw's twin brother – and 95 others.
In April, Tusk testified for eight hours in a separate intelligence probe by Warsaw's right-wing government that he described as a smear campaign to discredit him.