Turkish voters dealt the country’s once-invincible president, Recep Tayyip Erdogan, a sharp slap in the face Saturday. America should use the moment to nudge Ankara, a NATO member and erstwhile stalwart of the Atlantic alliance, back into the Western fold.

An opposition mayoral contestant in Istanbul, Ekrem Imamoglu, handsomely beat Mr. Erdogan’s favored candidate by a 9-point margin. This came after Mr. Erdogan’s Justice and Development Party, or AKP, canceled the results of an earlier election and ordered a redo because its candidate lost the first time around.

Democratic-minded Turks are hopeful this marks the beginning of the end of their Islamist leader’s increasingly oppressive rule. As politically weakened as he may be, however, Mr. Erdogan remains president until 2023, and therefore charged with conducting foreign policy.

The question for the United States and its European allies is: Can we leverage his unpopularity to force him to once more ally with Washington and NATO?

In recent years, Ankara has drifted away from the United States, striking up partnerships with some of the region’s worst actors, including Iran, the Muslim Brotherhood-backing emirate of Qatar and, most perilously for NATO, Russia.

A tilt back to the West can start with Turkey’s choice of air-defense systems. The Kremlin is expected to deliver its first installment of the S-400 air-defense battery to Turkey late in July.

The $2.5 billion S-400 deal with Moscow threatens NATO’s cohesiveness, as alliance members rely on interlinked Western-made air defenses. Turkey’s most ambitious joint project with America, the F-35 fighter jet, is also at risk from a Russian defense system designed to down American-made aircraft.

Back when it was a fully trusted NATO ally, Turkey was recruited to be one of a handful of countries participating in the development of the fighter jet of the future. Each country was to manufacture different components of the Lockheed Martin-made plane; each would receive F-35s to bolster its air force.

After Mr. Erdogan signed the S-400 deal with Russia, American lawmakers and national security officials started rethinking Turkey’s role. Turkish pilots’ training sessions to operate the F-35 were halted. Four jets that were to supposed to be delivered to Turkey remain in an Arizona base. The message to Ankara: You can have S-400s or the F-35 — but not both.

That message evidently didn’t register prior to Mr. Erdogan’s painful political setback Saturday. He insisted he wouldn’t renege on the Russia deal. Will he reevaluate now? Is he even capable of second thoughts? After all, he consolidated power by shutting down most opposition press and surrounding himself with sycophants.

He’s “very isolated. He’s advised by a narrow circle of yes-men, and the media feed him inaccurate info,” says a former Turkish parliamentarian, Aykan Erdemir, now with the Foundation for the Defense of Democracies in Washington.

So Mr. Erdogan won’t cancel the S-400 deal “as long as he continues to believe he can get away with it” and as long as he assumes a personal friendship with President Trump will get Turkey off the hook,” Mr. Erdemir adds.

Mr. Trump therefore must make things clear: Canceling the F-35 cooperation will cost Turkey some $12 billion, which dwarfs the $2.5 billion Russia deal. Designated as a regional service station for the F-35, Turkey would lose many local jobs. Mr. Erdogan’s political support would further erode.

Yes, some Turkish analysts argue that after the Saturday setback, Mr. Erdogan may lose faith in his ability to govern through the ballot box altogether. Instead of sobering up, he’ll turn to likeminded leaders in Russia, China, Venezuela, and the like.

Remember, though, the economy is one reason Mr. Erdogan lost Istanbul. Turkey’s growth rate has slowed significantly. Foreign investors are cautious as the lira loses value and Erdogan cronies take over industries. American pressure can further weaken the economy and force Mr. Erdogan to relent.

Last year, Washington used the Magnitsky Act to sanction top Ankara officials responsible for arbitrarily jailing Andrew Brunson, an American evangelical pastor who had lived in Turkey for two decades. Mr. Erdogan quickly caved and freed the pastor.

Washington could similarly punish officials involved in the S-400 project. When they meet this week, Mr. Trump should explicitly disabuse Mr. Erdogan of his mistaken belief that he can continue having arms deals with Russia and America at the same time.

Turkey’s location makes it a desirable strategic ally for world powers. Mr. Erdogan has ignored centuries of enmity between his country and Russia and seems willing to turn away from NATO and America to embrace Vladimir Putin.

As Mr. Erdogan’s grip on power loosens, America must warn Mr. Erdogan that such realignment is neither in Turkey’s interest — nor his.

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Twitter: @BennyAvni. From the New York Post.