TWO POLITICAL stories have dominated Turkey since the turn of the century: the steady consolidation of power by Islamist leader Recep Tayyip Erdogan and the emergence of the country’s Kurdish minority from decades of violent insurgency and repression to become a peaceful and accepted political force. On Sunday, those movements will collide in what may be the most important Turkish election since the country’s adoption of democracy 65 years ago.

Mr. Erdogan, who after three terms as prime minister took over the mostly ceremonial post of president last year, is seeking to lead his Justice and Development Party (AK Party) to a parliamentary majority large enough to rewrite the constitution. If he does, he will concentrate power in the presidency and, most likely, complete the transformation of his government into an autocracy. With its opposition still weak, the AK Party is considered likely to gain another majority in parliament; the question is whether it will gain the three-fifths necessary to introduce a new constitution with a referendum, or even the two-thirds needed to dispense with a popular vote.

The outcome will likely depend on whether the Kurdish People’s Democracy Party (HDP) succeeds in its unprecedented attempt to capture 10 percent of the overall vote and become a recognized bloc in parliament. If so, it will control enough seats to thwart Mr. Erdogan’s constitutional project, as its charismatic leader, Selahattin Demirtas, has promised. If it falls short of 10 percent — the highest threshold requirement of any electoral democracy — it will be shut out of the legislature and the AK Party may reach the three-fifths or two-thirds mark.

Polls suggest the HDP is at the threshold, which has so raised Turks’ fears of vote tampering that more than 50,000 have volunteered to serve as monitors. There is reason for concern: In recent years Mr. Erdogan has become increasingly reckless in his efforts to gain control of institutions, including the army and judiciary, and eliminate critics, especially in the media.

The president’s excesses were on display in the past two weeks in attacks on two leading newspapers. He accused one, Cumhuriyet, of espionage and said its editor should be jailed for life. Meanwhile, a prosecutor called for the arrest of the editor of the Hurriyet paper on ludicrous terrorism charges. Hurriyet’s supposed offense was publishing a headline about the death sentence for former Egyptian president Mohamed Morsi; Mr. Erdogan claimed the paper meant to imply he deserves the same.

The combination of a Kurdish loss and Erdogan triumph would be a double blow to Turkey and its Western alliances. NATO, which insisted that new members from Central Europe meet democratic standards, would find it hard to accommodate a Putin-esque Turkey; so would the European Union, which has been negotiating with Turkey about eventual membership. Meanwhile, the exclusion of Kurds from parliament could cause three years of peace talks with Turkey’s armed Kurdish insurgents, which are essential to the larger reconstruction of a war-torn region, to break down.

Most people outside Turkey’s southeast region normally would have little cause to support a small Kurdish political party. But a lot of liberal Turks and Western friends of the country will be rooting for Mr. Demirtas on Sunday.