BRUSSELS — Just as Poland secured a coveted place among the G20 group of leading economies, liberal Prime Minister Donald Tusk is arguing with nationalist President Karol Nawrocki over who gets to represent Warsaw at the top table.

Poland is one of Europe’s fastest growing economies and has NATO’s third-biggest military, but its diplomatic heft is being undermined by the clash between Tusk’s pro-EU camp and Nawrocki’s conservatives over who gets to speak for Warsaw on the global stage.

In their game of constitutional brinkmanship, Nawrocki’s presidential office is now moving to take control of preparations for Poland’s participation in next year’s G20 summit.

That drew a sharp public rebuke from Tusk.

“I will not allow the presidential palace to violate the constitution,” the prime minister said Thursday on the sidelines of an EU summit in Brussels.

The problem for Tusk is that U.S. President Donald Trump sees Nawrocki as an ideological ally, and it is Trump who will host the G20 summit at a golf resort he owns in Miami. Nawrocki scored a White House invitation in September and then managed to speak directly to Trump when Russian drones swarmed over Poland days later.

Trump’s administration made space for Poland by excluding regular G20 stalwart South Africa, but the opening quickly became a political battleground in Warsaw.

Tusk’s government proposed a joint approach, naming two “sherpas” to coordinate preparations: one from the cabinet and one from the presidential palace.

Nawrocki’s team headed straight to Washington to object, however, according to people familiar with the discussions. U.S. officials subsequently signaled they would deal only with the president’s envoy, Marcin Przydacz, who went on to present himself in Washington as Poland’s sole interlocutor for the summit.

Tusk did not block the trip and confirmed that Przydacz had received government briefing materials. But he warned that cooperation did not amount to consent, accusing the president’s camp of trying to shift authority “by force of facts, subterfuge and intrigue.”

“That is simply unacceptable and contrary to Poland’s interests,” he said.

Nawrocki’s office rejected Tusk’s version of events. The president’s spokesperson, Rafał Leśkiewicz, called the comments about violating the constitution a “perfidious manipulation,” arguing there was no constitutional drama to the episode.

“Those words in no way reflect the truth,” he said.

Leśkiewicz said the White House dealt with Nawrocki and his people because it “fully understands” that the Polish president is the country’s highest representative internationally, while the government’s role is to execute agreed positions.

“We would very much like Poland to speak with one voice,” Leśkiewicz said, accusing Tusk of turning what he described as routine coordination into an international quarrel and of struggling to accept the outcome of this year’s presidential election won by Nawrocki.

Similar clashes over who represents Poland are looming on the even more critical question of Ukraine. Warsaw is fuming it has been left out of crucial peace talks, but the internal rifts between Tusk and Nawrocki are unlikely to have tempted an invite to negotiations.

As Foreign Minister Radosław Sikorski put it recently, Poland can resemble “a car with two steering wheels” — perfectly drivable, but only as long as both drivers agree where they’re going.