The Turkish government accuses left-wing politician Andrej Hunko of supporting the Kurdistan Workers' Party. The group has been labeled a terrorist organization by the EU and the US.
Turkey has blocked a German parliamentarian from traveling to the country to be an election observer ahead of this Sunday's vote.
Andrej Hunko, a member of the German Bundestag for The Left party (Die Linke), was set to travel to Turkey before being notified by the Organization for Security and Cooperation in Europe (OSCE) that he would be denied entry.
Hunko told Germany's dpa news agency that the OSCE informed him of the Turkish ambassador's declaration shortly before his flight from Vienna to Ankara was to depart.
Hunko, who is also a member of the Parliamentary Assembly of the Council of Europe (PACE) for The Left party, participated as an OSCE election observer in the April 2017 constitutional referendum in Turkey.
Read more: Turkey elections: Can Erdogan really lose?
Turkey's opposition claimed irregularities in the referendum, which narrowly approved changing Turkey from a parliamentary into a presidential system.
Hunko criticized the "undemocratic and unfair conditions" under which that vote was held and suggested possible fraud was responsible for the narrow passage of the referendum.
Alleged ties to Kurdish PKK
Turkey accuses Hunko of being a supporter of the Kurdistan Workers' Party (PKK), a Marxist rebel group which the EU, Turkey and the United States consider a terrorist organization.
In an interview with German state broadcaster ARD in January, the Turkish foreign minister singled out The Left party and Hunko for being "fans of the terrorist organization PKK."
He also pointed to pictures of Hunko holding up a PKK flag at Kurdish demonstrations in Germany. Hunko has supported delisting the PKK as a terrorist organization and criticized a German ban against PKK symbols.
The German MP has rejected suggestions that he is close to the PKK.
"It is certainly absurd that the Turkish government acts as though I travel as an OSCE election observer in Turkey and make propaganda for the PKK," Hunko said Thursday. "That shows the Turkish government is nervous about the vote."
Polls indicate Turkish President Recep Tayyip Erdogan faces a close vote on Sunday. If he fails to get 50 percent in the first round, a second-round vote would likely see him face off against Muharrem Ince of the Republican Peoples' Party (CHP).
Erdogan's ruling Justice and Development Party (AKP) also faces the prospect losing its parliamentary majority, which it has held since 2002.
Opposition worries about fraud
The Turkish opposition and civil society organizations are planning to be out in force at election sites across the country to monitor voting amid concerns over fraud.
Sunday's election is the first under new electoral laws passed earlier this year by the AKP and ultranationalists.
Among the changes, ballots no longer need an official stamp to be valid.
Allegations of fraud last in year's referendum centered around the validity of some unstamped ballots, which the electoral commission deemed valid contrary to the election laws.
Other changes made to electoral law allow security forces to enter voting centers if requested by voters. The government says this is needed to prevent the PKK from pressuring voters, but the Kurdish opposition says that it will lead to security forces intimidating Kurds to vote for the AKP.
Another change in the law allows ballot stations to be moved to other districts for security, a tactic the Kurds say is designed to suppress the vote in the southeast.