Poles vote on Sunday in a presidential ballot that could place significant curbs on the power of their nationalist government. President Andrzej Duda is an ally of the ruling Law and Justice (PiS) party and if he loses, the opposition could force a big change in Polish politics. His closest rival is the liberal, centre-right mayor of Warsaw, Rafal Trzaskowski. Sunday's vote is expected to go to a run-off in two weeks' time. The election is being watched closely across Europe, as Mr Duda's allies in the Law and Justice Party (PiS) have frequently clashed with the EU over their controversial reforms to the judiciary and media. When President Duda this month condemned the promotion of LGBT rights as an "ideology" more destructive than communism, European Commission Vice-President Vera Jourova said it was "really sad" when European politicians in high office "decide to target minorities for potential political gains". The president's main rival has promised to repair ties with Brussels. Poland's president has the power of veto. If the next president were to use it, the governing party would not have the necessary majority in parliament to overturn it.