Tayyip Erdogan's ambitions for a swift move to strong presidential rule could suffer if dissidents in Turkey's nationalist opposition succeed in a party leadership challenge that could bolster their party's electoral support.

A bid by several hundred members of the Nationalist Movement Party (MHP) to challenge Devlet Bahceli, leader for much of the last two decades, at a special congress set for May 15 was approved by an Ankara court last month.

But rallying around its veteran leader, the MHP challenged that decision and an appeals court ruling is pending. Five candidates want to challenge Bahceli, including Meral Aksener, a 59-year-old woman who served as interior minister in the 1990s.

"I think this congress will happen on May 15. I think the vast majority of our great congress delegates will be there on that day," Aksener said in a statement that appeared to pay no heed to appeals court deliberations.

Uncertainty around Erdogan's role comes at a time of tension in NATO member Turkey as it faces Kurdish insurrection and spillover of violence from Syria. Western allies' concern for stability is tempered by reservations about what they see as his increasingly authoritarian attitude to media and opposition.

Some opinion polls suggest MHP support could rise above 20 percent if the leadership is changed, stealing seats from the ruling AK Party that it needs to change the constitution; but it could fall below the 10 percent threshold needed to enter parliament if Bahceli, 68, remained.

ELECTION THIS YEAR?

AK Party officials have said Erdogan could seek an early parliamentary election this year to capitalise on the MHP crisis if Bahceli remains in office and polls indicate its support falling below the threshold.

"If the appeals court decides the extraordinary congress should be held, we as the MHP leadership and delegates will be there out of our respect for the law," a senior MHP official close to Bahceli told Reuters, asking not to be named because of sensitivities within the party about challenging its leader.

"The dissidents used threats to gather signatures for the congress decision ... Even if they hold a so-called congress a significant portion of the delegates will not go to the hall."

Erdogan wants to replace Turkey's parliamentary democracy with a Turkish version of the U.S. or French system, where the elected head of state holds executive power. His opponents fear a stronger Erdogan presidency will increase authoritarianism and take Turkey further from Western democratic standards.

Turkey currently languishes in a form of political twilight.

Under the present constitution, the president should remain above party politics and everyday government; but Erdogan exerts strong influence through his popularity, and his prime minister, long in his shadow, has stepped down leaving the post for now unfilled and further weakening cabinet rule.

The AK Party, which Erdogan founded, has 317 lawmakers in the 550-seat assembly and needs at least 367 votes to change the constitution directly, or 330 to take the issue to a referendum.

Bahceli ruled out speculation he could back constitutional change. Speaking to his group in parliament, he also said creating an atmosphere for snap elections amounted to "treason".