Andrzej Duda and Donald Tusk. | Getty/Jeanette Minns Polish brothers at arms Donald Tusk and Andrzej Duda are set to engage in a running clash.

Donald Tusk may be the most senior office-holder in Europe, but that doesn’t keep him out of the political fray in his native Poland. With 43-year-old upstart Andrzej Duda newly installed as Polish President, the games have only just begun and the stakes are high: Duda could block Tusk’s nomination for a second 2.5-year term as European Council President in 2018.

Tusk’s notable absence from the August 6 swearing-in ceremony in Warsaw was not the first shot fired in a clash that has been in the making since the Law and Justice party’s Duda won May’s presidential contest, promising to revive a more nationalistic, socially conservative and Euroskeptic brand of Polish politics.

“Mr. Tusk has made no contact since the election. This information is confirmed by President Duda,” the President’s aide Marcin Kędryna told POLITICO. “Furthermore, Mr. Tusk was the only politician that did not send any congratulations letter.” Snap.

In fact, Tusk did congratulate Duda — but only in a tweet and only after praising the man Duda beat, Tusk’s colleague from the center-right Civic Platform party, Bronisław Komorowski.

It went unnoticed at the time, but while Tusk found time to congratulate the new Finnish Prime Minister Juha Sipilä and Britain’s David Cameron on their respective victories, he didn’t muster the energy to write to Duda, as European Commission President Jean-Claude Juncker and national leaders did.

Tusk’s office did not respond to repeated requests for comment on his relationship with Duda. They were, however, at pains to point out that the head of the Council did not attend Duda’s inauguration because “he was not invited by President-elect Duda.”

In the absence of a functioning relationship, Duda’s camp has been happy to stoke tensions. In May, his party colleague Ryszard Legutko, a member of the European Parliament, filed a complaint about Tusk with European Ombudsman, Emily O’Reilly.

Legutko, whose complaint was dismissed because he failed to first address it to Tusk and his team, accused the European Council president of violating impartiality rules governing EU employees by voicing support for Komorowski during his failed campaign.

It’s not hard to see why Tusk decided to intervene in the election, where Duda ran as a kind of ‘anti-Tusk.’

Burnt legacy

Michał Kolanko, founder and editor of 300polityka.pl, a political publication, said it boiled down to Duda positioning himself as the “voice of Poland in the EU,” arguing that the country “should be more assertive, that Poland should more directly defend the interests of its people.”

On the home front too, Duda and Tusk stand in stark contrast in both style and substance.

“Tusk, during his years as PM, constructed an informal so-called ‘warm water policy’ — providing only necessary and incremental fixes, avoidance of big reforms and ideology in policy. It worked, but was often derided ... Duda is trying to counter that in his rhetoric,” said Kolanko.

“He claims that he is ready to ‘fix the Republic’ and implement big, sweeping changes in almost all aspects of government,” said the analyst.

At least five substantive clashes loom on the political horizon:

1. Polish parliamentary elections in October: If Civic Platform’s Ewa Kopacz loses her bid to remain Prime Minister, it will amount to the burning of Tusk’s political legacy, coming just a year after his resignation from the post. The election campaign will be closely scrutinized for any sign that Tusk is positioning to rescue his legacy by running for Poland’s Presidency in 2020.

2. Global COP21 climate conference in December: Poland relies on coal, and sees EU policies to control the use of coal as antagonistic to Polish interests. Climate is Duda’s ‘two for the price of one’ political punch. By positioning himself as the EU’s problem child in climate negotiations, Duda can hurt Tusk by threatening to wreck the EU’s united front and win at home by securing costly sweeteners from Brussels. Duda can also use COP21 as a bargaining chip in discussions about Brexit, which is arguably the biggest challenge Tusk will face as Council President.

3. David Cameron’s EU reform package, ahead of Brexit referendum, 2016. Duda is no fan of red tape, but his economic policies — lowering the retirement age and a tough line on foreign firms — are more Orbán than Osborne, and definitely not Tusk. With Cameron wanting treaty change to derail the “ever closer union,” Duda is likely to needle Tusk by asking for Poland’s own climate opt-out. The pair will have to find common ground around free movement of people and labor, which is strongly supported across the political spectrum in Poland, but a concern for Cameron. In the U.K., Poles are the largest migrant population, with around 700,000 residing there.

4. Poland’s fraught euro entry. According to the Centre for European Reform’s Agata Gostyńska-Jakubowska, the euro will be the biggest clash: “Civic Platform and Law and Justice are like to have completely different views.” As Prime Minister, Tusk originally sought Polish entry into the single currency by 2012. That didn’t happen and now Duda wants to protect Poland as a leading ‘euro-out’ country. His potential Prime Minister and party colleague Beata Szydło has said euro entry is off the table until Polish wages catch-up with Western Europe. In contrast Tusk’s Civic Platform and Tusk as European Council President want Poland to join the euro once it meets the convergence criteria.

5. Donald Tusk’s re-appointment discussions, 2017-18. The working assumption in Brussels is that Donald Tusk wants re-appointment in 2018 to serve a full five years as European Council President. That means Tusk’s fate is in his opponent’s hands: he needs Duda’s vote, or the re-appointment can be blocked. That itself would be a huge risk for Duda — undermining the only Pole to hold one of the top EU posts. Blocking Tusk would almost certainly guarantee further domestic interventions, as Tusk would then have full-time possibilities to begin a campaign to oust Duda.

This story was updated to correct the position of Marcin Kędryna.