ISTANBUL (Reuters) - Dissidents in Turkey's nationalist MHP opposition party took a step closer on Sunday to ousting their veteran leader, a move which could potentially derail President Tayyip Erdogan's drive for greater powers under a new constitution.

The dissidents are trying to oust Devlet Bahceli, a dour 68-year-old who has led his party since 1997 but has seen support among voters ebb despite growing nationalist sentiment in Turkey.

At an extraordinary congress on Sunday attended by 657 delegates, the dissidents managed to amend a party bylaw that will allow them to challenge Bahceli's leadership at a meeting set for July 10.

They hope to replace Bahceli with Meral Aksener, a 59-year-old woman seen by pollsters as having the mettle needed to revitalize the MHP and increase its support. That could make Erdogan's ambitions harder to push through parliament.

If Bahceli survives the challenge - and many expect him to challenge the bylaw amendment in the courts - pollsters and political analysts say he could lead his party to a crushing defeat at the next election, exiting parliament and freeing Erdogan's hand to rewrite Turkey's constitution.

Erdogan and his supporters say Turkey needs a strong executive presidency to help steer the country through its big security and economic challenges. Under the current constitution, Turkey's president is a largely ceremonial post.

Critics say Erdogan is behaving in an increasingly authoritarian way, undermining Turkey's constitutional checks and balances and clamping down on media freedoms in the European Union candidate nation. Erdogan rejects the accusations.

(Reporting by Ercan Gurses and Seda Sezer; Editing by Gareth Jones)