ANKARA, Turkey (AP) — Turkey’s main opposition party Monday prepared to contest the results of a landmark referendum that gave a narrow victory to President Recep Tayyip Erdogan’s long-time plans to greatly expand the powers of his office.

Turkey’s electoral board confirmed the “yes” victory in Sunday’s referendum and said the final results would be declared in 11-12 days. The state-run Anadolu Agency said the “yes” vote stood at 51.41 percent, while the “no” vote was 48.59 percent.

The margin fell short of the sweeping victory Erdogan had sought in the referendum. Nevertheless, it could cement his hold on power in Turkey and is expected to have a huge effect on the country’s long-term political future and its international relations.

“I suspect the result was narrower than what Erdogan expected,” said Howard Eissenstat, associate professor of Middle East History at St. Lawrence University in the U.S. “Erdogan has ruled with a narrow victory before. He does not see a narrow victory as anything less than a mandate. His tendency has been to not to co-opt the opposition but to crush it.”

The 18 constitutional amendments that will come into effect after the next election, scheduled for 2019, will abolish the office of the prime minister and hand sweeping executive powers to the president.