By Ercan Gurses and Seda Sezer

ISTANBUL (Reuters) - Turkey's nationalist opposition has expelled a member who attempted to oust its veteran leader, the party said on Thursday, jettisoning a rebel seen by some as capable of chipping at the power of President Tayyip Erdogan.

Opinion polls had suggested the removal of Devlet Bahceli as leader of the Nationalist Movement Party (MHP) could broaden its support and weaken Erdogan's efforts to gather backing for a full presidential system with strong executive powers.

The MHP central disciplinary council said in a statement it had taken the unanimous decision to expel Meral Aksener. Disciplinary investigation continued against other dissidents.

Party rebels blame slipping support for the party on Bahceli, 68, whom they see as a dour figure unable to tap into nationalist sentiments that have buoyed Erdogan.

"Suffice to say this move affirms Bahceli's control over the MHP, albeit hopes of the modernization of the MHP now probably go down the drain, and the dominance of the AKP will likely be further assured," said Nomura strategist Tim Ash in a note.

Erdogan, who founded the ruling AK Party, remains Turkey's most popular politician. His popularity has been further bolstered by the failed July 15 coup, when an element of the military attempted to overthrow the government.

At an extraordinary congress in June, dissidents including Aksener managed to amend an MHP bylaw that would allow them to challenge Bahceli's leadership.

But following the extraordinary congress, an election body barred opposition nationalists from holding a July 10 vote to replace him, disrupting a process that could have picked a stronger challenger to President Erdogan.

Erdogan and his supporters have said Turkey needs a strong executive presidency to help steer the country through its big security and economic problems. Under the current constitution, Turkey's presidency is a largely ceremonial post, though Erdogan exerts broad control by dint of his personal popularity.

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 Daren Butler, Writing by Seda Sezer; Editing by Ece Toksabay and David Dolan; editing by Ralph Boulton)