When leaders travel to countries that are in the midst of an election campaign, their main concern is generally deciding which party to support - or going to great lengths to remain neutral.

But before she has even landed, Chancellor Angela Merkel's visit to Turkey, scheduled for Sunday, has been roundly criticized by German opposition parties.

In a parliamentary debate, Greens politician Claudia Roth accused the chancellor of setting foot on a "mine zone" and supporting Turkish President Recep Tayyip Erdogan's election campaign.

Sevim Dagdelen, a member of parliament of Turkish origin, who is in the Left party asked Merkel not to talk with Erdogan.

"Erdogan should no longer be this government's partner," she said. If Merkel meets with the "despot" then she will show that the "victims of Ankara do not matter to her."

Officially, however, Erdogan is not even in the running. The parliamentary elections set for November 1 are to elect a new parliament and the president is supposed to be non-partisan, according the Turkish constitution. But Erdogan has intervened in the election campaign and his policy of escalation towards the Kurds has made the elections also a referendum on him. The situation in Ankara is tense after last week's attack on a peace demonstration. Bomb blasts killed nearly 100 demonstrators - mainly supporters of the opposition.

The fact that Merkel wants to discuss stopping the movement of refugees to Europe - not domestic Turkish politics - does little to ease the fear that her visit may be exploited by Erdogan for political purposes.

Words of caution have also been spoken by the governing parties. Hans Peter-Uhl, a member of Christian Social Union (CSU), the Bavarian sister party of Merkel's ruling Christian Democratic Union (CDU), defended the chancellor by accusing the opposition of harming German interests.

Andreas Nick (CDU) stressed the fact that Turkey remains "a strategic partner despite all criticism." But one of his fellow party members, foreign policy expert Roderich Kiesewetter, had less diplomatic words for the Turkish president's "poisoned offer" to help Europe cope with the refugee crisis, which he said Merkel should not accept. In exchange for his cooperation in dealing with refugees, Erdogan expects political concessions in Syria and in the conflict with the Kurds.

Niels Annen, a member of the Social Democratic Party, which governs with the CDU, said he believes the visit is "absolutely correct," but added that the chancellor should also meet with representatives of the opposition to avoid giving impression that she backs Erdogan's aspirations in the elections.

Sharper criticism for Merkel came from SPD politician Dietmar Nietan, who called the idea "ridiculous." Green politician Inge Höger said the visit to Turkey was "cynical and ridiculous." Green Party Co-chairman Cem Özdemir said the Turkish president himself was a "personified cause for flight."