By Burcu Karakas and Umit Ozdal
ISTANBUL/DIYARBAKIR (Reuters) -Kurds opposed to President Tayyip Erdogan fear victory for him in Turkey's presidential election could reinforce a crackdown the state has been waging against them for years, alarmed by a surge in nationalist rhetoric ahead of Sunday's vote.
Kurds, who make up around a fifth of Turkey's population, have been seen as potentially crucial to the opposition's hopes of ending Erdogan's 20 years in power - a reign in which he first courted but then cracked down hard on Kurdish groups.
But the momentum is with Erdogan going into Sunday's vote after the first round gave him a lead over the opposition's Kemal Kilicdaroglu, who trailed despite the backing of six parties and the endorsement of the pro-Kurdish HDP party.
For some Kurdish voters, the stakes couldn't be higher as Erdogan dials up his nationalist tone in a bid to win more votes ahead of the runoff.
"Voting is a matter of life and death now. Erdogan hardened his stance on Kurds during the election campaign," said Ardelan Mese, 26, a cafe owner in the mainly Kurdish city of Diyarbakir in the southeast who is urging his friends to vote.
"I can't imagine what he will be capable of after declaring victory."
The HDP won 61% support in Diyarbakir in the May 14 parliamentary election, while Erdogan's AKP got 23%. Nationwide, HDP support was 8.9%.
The issue of HDP support for Kilicdaroglu has this week been complicated by his deal with an anti-immigrant party that HDP said was "against universal democratic principles". But the HDP reaffirmed its support on Thursday without naming him.
In his early years in power, Erdogan expanded political and cultural rights for Kurds, a stateless group scattered between Turkey, Iran, Syria and Iraq.
He removed restrictions on using the Kurdish language and oversaw a peace process with the Kurdistan Workers Party (PKK), which took up arms against the state in 1984 and is viewed as a terrorist group by Turkey and its Western allies.
But after a ceasefire collapsed in 2015, he changed course, with the authorities waging a crackdown that led to the arrest of thousands of HDP members, typically on charges of militancy, with many of its lawmakers and mayors unseated and jailed.
Seizing on the HDP's support for Kilicdaroglu, Erdogan has repeatedly accused the opposition of siding with terrorism. The HDP denies accusations of militant ties.
Erdogan also repeatedly drew attention to a doctored video to accuse Kilicdaroglu of ties to the PKK, which has been waging an insurgency in which more than 40,000 people have been killed.
Kilicdaroglu called the accusation slanderous.
But Erdogan's stance has won the endorsement of Sinan Ogan, a hardline nationalist who came third in the first round. Ogan has said the endorsement was based on a principle of "non-stop struggle (against) terrorism", referring to pro-Kurdish groups.
The sloganeering marks an effort to tap deep-running nationalist sentiment - a powerful force in Turkish politics which has been hardened by factors including years of hostilities with Kurdish militants and the influx of millions of refugees from Syria since war began there in 2011.
Critics say Kilicdaroglu has also struck a more nationalist tone since a disappointing first round, vowing to repatriate all refugees. Turkey hosts some 5 million migrants, of whom 3.4 million are Syrians, according to Interior Ministry data.
He has also noted Erdogan "sat at the table with terrorist organisations" - a reference to the failed peace process.
Tayip Temel, a senior HDP official, expressed fear that Erdogan had achieved his desired goal through "black propaganda" alleging links between Kilicdaroglu and terrorists.
"Erdogan's victory will consolidate one-man rule and pave the way for horrible practices, bringing completely dark days for all parts of society," Temel, HDP deputy co-chair, said.
He also noted Kilicdaroglu's switch to "a more nationalistic discourse" which he said could cost him some Kurdish support.
Reha Ruhavioglu, director of the Diyarbakir-based Kurdish Studies Center, believes many Kurdish voters have lost the will to vote ahead of the runoff, blaming the shift in the tone adopted by Kilicdaroglu.
"Demotivation stems from the CHP's political discourse which shifted from reconciliation to security politics. The HDP should motivate its base by saying, 'If Kilicdaroglu loses, we lose too'," he said.
The HDP contested the elections under another party's banner due to a court case threatening to ban it for alleged links to Kurdish militants, which it denies.
Critics fear a statement by Interior Minister Suleyman Soylu on Sunday accusing a group of Kurdish lawyers of PKK links could be a sign of a new crackdown ahead.
Nahit Eren, head of the Diyarbakir Bar Association, told Reuters the statements could prompt the authorities to open a new "terrorism investigation".
(Reporting by Burcu Karakas; Editing by Tom Perry, Daren Butler and Nick Macfie)