Turkish President Recep Tayyip Erdogan has called Germany's ruling politicians "enemies of Turkey" who deserve to be rejected by German-Turkish voters.
Germany will hold a general election on 24 September, and about one million ethnic Turks living in Germany can vote. A majority of them backed Mr Erdogan in an April referendum.
"The Christian Democrats [CDU], SPD [Social Democrats], the Green Party are all enemies of Turkey," he said.
German ministers protested angrily.
Foreign Minister Sigmar Gabriel said Mr Erdogan's comments were an "unprecedented" act of interference in Germany's sovereignty.
Mr Erdogan has lashed out at Germany before, yet the two countries are major trade partners and allies in Nato.
He was furious that the German government refused to let some of his allies campaign for him in Germany before the April vote, which paved the way for him to get sweeping new executive powers. That refusal, he said, was "Nazi-style" behaviour.
Tensions increased after the abortive coup attempt against Mr Erdogan in July 2016, during which at least 240 people died.
President Erdogan blamed the network of US-based cleric Fethullah Gulen over the coup plot, and accused Germany of protecting Gulenists. The cleric has denied any role in the plot.
Mr Erdogan conveyed his message to German Turks via reporters in Istanbul after Friday prayers.
"Give necessary support to political parties that do not engage in enmity against Turkey.
"It is not important whether they are the first or the second party. In a way this is a struggle of honour for all my citizens living in Germany," he said, implying that voters should back far-right or far-left parties.
Chancellor Angela Merkel's centre-right CDU has been governing in coalition with the Mr Gabriel's centre-left SPD. Opinion polls suggest the CDU has a strong lead over the SPD.
The Turkish diaspora in Germany is estimated to number about three million people. "I think they will be giving the necessary lesson to those parties at the ballot box," Mr Erdogan said.
More than 50,000 people have been arrested in Turkey since the coup plot, including hundreds of journalists, opposition politicians, academics and activists.
Mr Erdogan's ruling AK Party has also purged the armed forces, judiciary, police and education sector, sacking more than 140,000 people.