“Erdogan has gained the most from his ability to persuade — some would say manipulate — the (U.S.) president,” Josh Rogin said in an op-ed.

Examples include northern Syria, where the U.S. president briefly permitted a Turkish military operation against the Kurdish-led autonomous administration, and the U.S. State Department’s investigation into a Turkish bank accused of breaking sanctions on Iran, which Trump allegedly sought to quash.

However, Rogin said the biggest favor Trump has granted Erdogan is his refusal to impose sanctions on Turkey over the purchasing of the Russian-made S-400 missile defense system.

In contrast, Trump’s presidential rival Joe Biden is unlikely to stop the sanctions push, which “is about to gain even more momentum on Capitol Hill”, Rogin said.

Biden has indicated that he would take a more assertive stance on Turkey, telling the New York Times that Erdogan “has to pay a price”.

“The animosity between Erdogan and Biden runs deep,” Rogin said citing an alleged incident after Biden met opposition figures in Turkey.

“Erdogan was so incensed that he barged into a private meeting between their spouses and told Jill Biden that she should get her husband under control.”

With Biden leading the polls, Erdogan has reason to worry ahead of the November 3 election, Rogin said.

“If Biden wins, Erdogan’s free ride is surely over.”

Reporter’s code: 50101