Turks are heading to the polls in a crucial parliamentary election that will determine whether ruling party lawmakers can rewrite the constitution to bolster the powers of President Recep Tayyip Erdogan.

Erdogan himself is not on the ballot, but the election is effectively a referendum on whether to endow his office with extraordinary powers that would significantly change Turkish democracy.

Erdogan's ruling Justice and Development Party, or AKP, is expected to win significantly more votes Sunday than any opposition party. But AKP must win a supermajority of the 550 seats in parliament to change the constitution.

All eyes will be on the results for the main Kurdish party, HDP. If it crosses a 10 percent threshold for entering parliament as a party, that would extinguish AKP's constitutional plans.