Turkey’s opposition candidate won the mayoral election rerun in Istanbul on Monday, handing longtime Turkish President Recep Tayyip Erdoğan a stinging defeat and raising questions over whether or not this could spell the beginning of the end of his rule.

Harold Rhode, a former adviser on Islamic affairs and specialist on Turkey in the U.S. Department of Defense and now a distinguished senior fellow at the Gatestone Institute think tank, told JNS that this is “a huge blow” for Erdoğan as Istanbul is the economic and cultural center of the country.

When asked how Erdoğan could let the opposition win seeing as he has so much power, Rhode said, “In the first election [back in May], Erdoğan’s AKP [party] thought they had it, and then they found an excuse for a re-vote [when they lost].”

Ekrem Imamoğlu of the opposition Republican People’s Party (CHP) got about 54 percent of the vote, more than in the previous victory three months ago, Reuters reported. The previous election was canceled after protests by Erdoğan’s AKP claiming voting irregularities.

“Erodoğan figured he could manipulate the re-vote, but there was around a 800,000 vote difference and [there was] not much he could do to change that,” said Rhode. As fallout from this loss, within Erdoğan’s Islamist AKP “there are now rumblings among factions that may consider breaking off from the party,” added Rhode.

If this were to occur, it could become a serious threat to Erdoğan.

Rhodes said that Erdoğan, like other politicians in the region, may have overstayed his welcome, with the people now turning against his dictatorial rule.

Meanwhile, Turkish Foreign Minister Mevlut Cavusoğlu said on Monday that Turkey had purchased the Russian S-400 missile-defense system despite pressure by the United States to cancel the deal.

“No matter what sanctions decision, no matter which statement comes from the United States, we have already bought the S-400s,” Çavuşoğlu told reporters, according to the Turkish Hürriyet Daily News. “Now we are talking about when the S-400s will be delivered to Turkey. It is not possible for us to give up on the purchase of the S-400.”

The United States warned on Thursday that sanctions against Turkey for planning to acquire the S-400 Russian missile-defense system is a “very viable” option, despite a threat by Erdoğan to retaliate were that to happen.