"I am in favour of backing a joint candidate" Selahattin Demirtas, who ran against Erdogan twice, told AFP through a lawyer from his jail in the western city of Edirne.

The Peoples' Democratic Party (HDP) -- parliament's third-largest -- faces the threat of being banned ahead of polls in which Erdogan will seek to extend his rule into a third decade.

Erdogan portrays the HDP as the political wing of outlawed Kurdish militants who have been waging a decades-long insurgency against the Turkish state.

The party says it is being singled out for standing up for Kurdish rights and resisting Erdogan's crackdown on civil liberties.

Turkey's top court is expected to rule on a prosecutor's request to shut it down in the coming months.

The party's legal problems add a new layer of uncertainty to the parliamentary and presidential polls -- widely viewed as Turkey's most important in generations.

The HDP has been excluded from a six-party opposition alliance now trying to agree on a single candidate to run against Erdogan.

But after securing 12 percent of the vote in 2018 elections, the HDP's future could prove decisive in what promises to be a tight race.

- 'Compromise' -

Demirtas's second presidential challenge came from behind bars, where he has languished since 2016 on a myriad of charges, some of them terror-related.

The 49-year-old denies them all and the European Court of Human Rights agrees, repeatedly calling for his release.

Demirtas has been convicted on some counts since the last election, making him ineligible to run again.

But the party's co-chairwoman, Pervin Buldan, suggested this month that the party should still field its own candidate, even without its brightest star.

Demirtas conceded that Buldan might ultimately get her way.

"At this stage, it seems more likely that the HDP will nominate its own candidate," he said.

But a "compromise with the HDP through negotiations" could still produce a joint candidate representing Turkey's entire opposition -- including the Kurds, he said.

Buldan has been frustrated by the six-party alliance's refusal to openly court the HDP vote.

Some of the six are staunchly nationalist and wary of close association with the Kurdish cause.

- Gaining acceptance -

An alliance with the HDP helped the opposition secure mayoral victories in Istanbul and Ankara in 2019, wresting Turkey's two main cities from Erdogan's ruling party for the first time in 25 years.

The party's closure before the election could see traditional HDP voters either stay at home or back the joint opposition candidate.

Turkey has shut down other pro-Kurdish parties for alleged links to the outlawed Kurdistan Workers' Party (PKK), which is recognised as a terror group by Ankara and its Western allies.

But Kurds have quickly regrouped in the past, creating new parties that slowly gain broader acceptance across society, particularly for their socially liberal views.

Demirtas sounded certain that a new party would rise up if the HDP, which he co-chaired from 2014 to 2018, was dissolved.

"The HDP electorate will not be left without an option or a party," he said.

The vote's outcome also has personal implications for Demirtas.

Turkey's main opposition leader, Kemal Kilicdaroglu, has pledged to release political prisoner should he win.

Erdogan himself might announce an amnesty as a gesture of good will after the polls.

Demirtas, who faces 142 years in prison if convicted of all the charges, remained defiant.

"I never expect forgiveness from anyone, especially not from Erdogan," he said. "I am the one who should be pardoning Erdogan, he is the one who is guilty."