Turkey on Sunday voted in a momentous election that could extend President Recep Tayyip Erdogan's 21-year grip on power or put the mostly Muslim nation on a more secular course.

The presidential and parliamentary ballot has turned into a referendum on Turkey's longest-serving leader and his party.

It is also the toughest of more than a dozen that Erdogan has confronted -- one that polls suggest he might lose.

The 69-year-old has steered the nation of 85 million through one of its most transformative and divisive eras in the post-Ottoman state's 100-year history.

Turkey has grown into a military and geopolitical heavyweight that plays roles in conflicts stretching from Syria to Ukraine.

The NATO member's footprint in both Europe and the Middle East makes the election's outcome as critical for Washington and Brussels as it is for Damascus and Moscow.

But Erdogan's first decade of economic revival and warming relations with Europe was followed by a second one filled with social and political turmoil.

He responded to a failed 2016 coup attempt with sweeping purges that sent chills through Turkish society and made him an increasingly uncomfortable partner for the West.

The emergence of Kemal Kilicdaroglu and his six-party alliance -- a group that forms the type of broad-based coalition that Erdogan excelled at forging throughout his career -- gives foreign allies and Turkish voters a clear alternative.

Polls suggest the 74-year-old secular opposition leader is within touching distance of breaking the 50-percent threshold needed to win in the first round.

A runoff on May 28 could give Erdogan time to regroup and reframe the debate.

But he would still be hounded by Turkey's most dire economic crisis of his time in power and disquiet over his government's stuttering response to a February earthquake that claimed more than 50,000 lives.