As Turkey considers granting Recep Tayyip Erdogan sweeping new powers, the crucial swing voters may be the country's long-oppressed ethnic minority.

ISTANBUL — Turkey’s most outspoken opponent of a referendum that promises to transform this NATO ally is sitting out the campaign behind bars.

Selahattin Demirtas, an ethnic Kurd who had rallied a national coalition with his pledge to thwart President Recep Tayyip Erdogan’s bid for vast new powers, has been largely silenced, arrested in November 2016 for suspected ties to militants. A dozen more lawmakers from his Peoples’ Democratic Party (HDP) are also behind bars.

Demirtas’s political career has fallen victim to the surging violence between Kurdish insurgents and the Turkish state. A peace process with the armed Kurdistan Workers’ Party (PKK) that Demirtas helped broker has collapsed, cities across the mainly Kurdish southeast lie in ruins after a reignited insurgency, and Erdogan is poised to clinch the nationwide referendum on constitutional amendments that would transform Turkey from a parliamentary system into a presidential one — and could keep him in office past 2030.

If the “yes” vote wins out, Erdogan would no longer occupy a largely ceremonial role as president, but rather would run the military and the rest of the executive branch, exert greater control over judicial appointments, and, critics argue, effectively shape the legislature, because he would be able to resume his leadership of the ruling Justice and Development Party (AKP) and select its candidates for parliament.

The referendum campaign is tight, with recent opinion polls showing the “yes” campaign at 51 to 53 percent. A majority of Turkey’s Kurds backed Demirtas’s left-wing HDP in the June 2015 general election, and now their vote could prove decisive in the April 16 plebiscite.

Erdogan’s message to Turkey’s 58 million voters is a potent one: Only when he has complete control can he crush one of Europe’s longest-running insurgencies, which has claimed more than 40,000 lives. The PKK, which took up arms in 1984, is classified as a terrorist organization by the United States and the European Union.

“The coming months, with God’s permission, will be spring for Turkey and the Turkish people and a black winter for terrorists,” Erdogan said in an April 3 speech in the nationalist stronghold of Trabzon. He has hinted that he may pursue the Kurdish rebels into Iraq and Syria, where PKK-linked militias are battling the Islamic State with the U.S.-led coalition.

But Erdogan has tailored a different case for Kurdish voters. Under tight security in Diyarbakir, Demirtas’s hometown and the largest city in the impoverished southeast, Erdogan on April 1 told supporters that a “yes” vote will deliver peace and prosperity.

“I am personally the guarantor of the rights you possess, your freedoms, the economic development you need,” he said.

Erdogan has traditionally attracted support from about half of Kurds — conservatives grateful for the expansion of cultural rights during his rule and a peace process he launched with the PKK until it shattered in 2015.

Kurds make up about a fifth of the voting public, or roughly 10 million voters. If traditional Erdogan supporters among them return to his fold, it is likely to push the “yes” vote above the simple majority he needs. On the other hand, if the nearly 1 million Kurdish voters who abandoned the AKP for the HDP in the June 2015 election stick with the “no” camp, they could jeopardize Erdogan’s campaign.

“Embracing the Kurds as equal citizens has been a key part of Erdogan’s political legacy,” wrote Ibrahim Kalin, Erdogan’s press secretary, in the Daily Sabah newspaper on April 3, citing the loosening of restrictions on the Kurdish language and millions of dollars of investment in Kurdish towns and cities. “The polls suggest that Kurdish voters support the constitutional referendum in higher numbers than expected.”

In a rare prison interview, where Demirtas submitted responses to written questions through lawyers, he told Foreign Policy that Erdogan will struggle to recapture Kurdish votes after the failure of peace negotiations and the ensuing crackdown. He predicted that a “significant proportion” of those Kurds who previously voted for the AKP would now vote “no.”

“The hard-line policies of the AKP and the suspension of democracy, human rights and rule of law have been rebuffed by Kurds as they distance themselves from the AKP,” he said.

Erdogan’s battle to consolidate power comes after a botched military coup in July 2016 that sought to overthrow him and killed more than 240 people. A subsequent state of emergency has allowed him to rule by decree, and some 113,000 people have been purged from the civil and security services and nearly 50,000 people imprisoned. Though not implicated in the attempted putsch, Demirtas and the other lawmakers faced a flurry of criminal charges in its wake. (The HDP lawmakers’ parliamentary immunity had been revoked by their fellow lawmakers in May, prior to the coup attempt, at Erdogan’s urging.) Turkish authorities also jailed thousands of rank-and-file members of the HDP and an affiliated party and snatched control of 82 municipalities that their elected mayors ran.

Election monitors from the Organization for Security and Cooperation in Europe reported “no” campaigners have faced a de facto media ban, police interventions, and scuffles at events. Emergency rule raised concerns “about whether appropriate conditions are in place to hold a referendum.”

Other observers also warned that the playing field was tilted in favor of Erdogan’s campaign.

“Everything is skewed in favor of the ‘yes’ vote because of the silencing of the media, the removal of key political figures like Demirtas,” said Emma Sinclair-Webb of Human Rights Watch.

“It is appalling that the population has to make this choice about the country’s future under a state of emergency when many rights and freedoms are restricted. In this polarized atmosphere, there cannot be an open discussion.”

Erdogan likens such criticism to an “open campaign” by the West for a “no” vote. This race has seen considerably less violence than recent general elections.

Demirtas, who turned 44 in prison this week, said he was “absolutely” certain his arrest was designed to cripple opposition to the constitutional changes. Instead of drumming up support for the “no” vote, Demirtas now spends his days writing or exercising in a small yard adjacent to his cell in a prison situated near the Greek and Bulgarian borders in the former Ottoman capital of Edirne. He sees his wife, Basak, once a week and his daughters, aged 11 and 13, twice a month. The HDP’s co-leader, Figen Yuksekdag, was detained with Demirtas, and in February a court convicted her of issuing “terrorist propaganda,” stripping her of her seat in parliament.

“We have been held unlawfully for five months, and we still don’t know when our prosecution will begin,” Demirtas said. “We are confronted not with a judicial process but extrajudicial enforcement.”

AKP officials deny this, accusing the HDP of serving as a PKK mouthpiece. The interior minister, who oversees the police force, said last month that Kurdish-run municipalities had funneled money to the PKK and recruited its foot soldiers.

Demirtas, a former rights campaigner, denies that he or his party is tied to the PKK and has long advocated for a negotiated settlement to the conflict. He ran against Erdogan for the presidency in 2014 in a quixotic bid meant to test the waters for his party to challenge the general election.

In the June 2015 general election, Demirtas’s electoral coalition emerged as a force to be reckoned with. He galvanized voters with a pledge to block Erdogan’s ambitions for greater power and with a progressive party platform that included equal rights for women and respect for the environment, winning over liberal Turks once wary of his roots in a Kurdish nationalist movement. Pious Kurds who had previously backed Erdogan flocked to Demirtas, in part because he had come to play an important role in the government’s two-and-a-half-year peace talks with Kurdish rebels. Demirtas ran on a mandate of seeking a political settlement that would be fair to both sides of the three-decade war. Together, these voters elected a record 80 HDP deputies to the 550-seat parliament, depriving the AKP of single-party rule for the first time.

But Demirtas’s political rise was soon disrupted by renewed fighting between the Turkish government and the PKK, the deadliest in two decades. Some 2,000 people were killed and 500,000 displaced in 2015 and 2016, according to the United Nations. A PKK offshoot claimed responsibility for a series of bomb attacks that killed more than 100 people in western Turkish cities. Majority Kurdish towns in a half-dozen provinces were wrecked during security operations to root out PKK militants who used residential streets for trench warfare.

In areas like Sur, a historic neighborhood in the southeastern city of Diyarbakir, the militants made far-fetched declarations of autonomy. Sur’s Roman-era walls and ancient churches and mosques, listed by UNESCO as a World Heritage site, suffered irreparable damage.

Demirtas condemned the violence but reserved most of his ire for Erdogan. Some analysts see that decision as his undoing.

“Demirtas walked into the trap set for him by Erdogan and the PKK,” said Levent Gultekin, a writer and former conservative. “He failed to use the mandate he was given by voters to stand up to the PKK. Faced with that challenge, the HDP choked.”

For his part, Demirtas did not express regret for his party’s response to the eruption of violence.

“Despite all that has happened, we are not asking for mercy or forgiveness from anyone, because we committed no crime. We are being held here because a crime is being committed against us,” he said. “Sooner or later, those who stand for peace and democracy will win.”

Unwilling or unable to help end the bloodshed, Demirtas bitterly disappointed both Turks and Kurds who were drawn to his conciliatory campaign promises, including farmer Mustafa Celik, 43.

In a remote village outside of Diyarbakir, Celik, an ethnic Kurd, named his eighth child Evet, which means “yes” in Turkish, when she was born in January.

“We sent the HDP to parliament to bring peace, but they stood with the PKK. Who elected them: the people or the PKK?” Celik said. “As a Kurd, I am sad Demirtas is in jail. But if Erdogan wins the referendum, peace will return.”