By Orhan Coskun and Ercan Gurses
ANKARA (Reuters) - As he battles to cling to power after nearly two decades as leader of Turkey's nationalist opposition, Devlet Bahceli is arguably more important to the future of Turkish politics than ever before.
The dour 68-year old, who took over the leadership of the Nationalist Movement Party (MHP) in 1997, has done little to move it from the political margins ever since, its popularity declining over the years despite growing nationalist sentiment.
But as he fights an increasingly bitter leadership challenge in the twilight of his career, Bahceli has been thrust center stage in the main political saga of the day: President Tayyip Erdogan's effort to accumulate greater powers.
Should he stay, support for the MHP could drop to the point that it is forced from parliament, all but guaranteeing Erdogan the backing he needs for a referendum on changing the constitution and forging a stronger presidency.
Should he be ousted, his main rival Meral Aksener could take over, 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.
"Some time ago a Bahceli victory was seen as definite, but now every possibility is there," said one source close to the ruling AK Party, which was founded by Erdogan and is watching the leadership battle in the MHP with increasing trepidation.
Bahceli appears on the back foot. His supporters have tried to block a special party congress called for June 19 by several hundred MHP members at which he could be ousted, but Turkey's court of appeals has ruled it can go ahead.
Hakan Bayrakci, head of pollster SONAR, said Aksener, who served as interior minister in 1996-97, would have the support of around 750 of the 1,241 delegates, enough to force him out.
But Bahceli's allies are fighting a rearguard action, launching an investigation by the MHP's Central Disciplinary Board - which has the power to ban members from the party - into Aksener and three other challengers.
"Our dissident colleagues deserve a punishment for their activities against the party. The board may prevent their candidacy (in a leadership bid)," said one senior MHP official loyal to the veteran leader.
Bahceli himself has said the party can go ahead with a congress in July at which he will stand again for the leadership. By then, the disciplinary board may have ruled, but a second Bahceli ally said he would still be taking a risk.
"He risks losing his seat by taking the decision to go to the congress," the ally said, speaking on condition of anonymity for fear of undermining Bahceli's position.
Pollsters estimate that should Bahceli hang on, support for the MHP could drop to below the 10 percent threshold needed for representation in parliament, down from a peak of 18 percent in 1999 and from 12 percent at the last polls in November.
That could prompt Erdogan to call a parliamentary election and strengthen the majority of the ruling AK Party by enough to guarantee support for a referendum on constitutional change.
Erdogan served as prime minister until 2014 when he became Turkey's first directly elected president, with the hope of quickly transforming what had been a largely ceremonial role into one with executive powers.
Even without constitutional change, he has already overhauled the role, regularly chairing cabinet meetings in his new palace and largely ignoring the constitutional requirement for him to renounce his affiliation with the AKP.
Of all of Turkey's opposition parties, the MHP is the most likely to have some sympathy for Erdogan's ambitions. The AKP also has a deeply nationalist strain, and the two parties have of late been on the same page when it comes to crushing by force a Kurdish militant insurgency in Turkey's southeast.
Askener is strongly opposed to an executive presidency. But some AKP officials hope that, if she takes over the leadership of the nationalists, angry Bahceli supporters in parliament might retaliate by helping Erdogan enact his changes.
"If Bahceli loses and Aksener gets in ... at least 15 MHP deputies who are unhappy with this may support the presidential system. We have this expectation," a senior AKP official said.
But such backing from Bahceli supporters is far from guaranteed. Speaking in parliament late last month, Bahceli said a presidential system would inevitably lead to despotism and be costly for Turkey in the long-run.
A third senior MHP official said the ruling party was exploiting the leadership battle by suggesting factions within the MHP might support the executive presidency, in a bid to deepen divisions.
It was a strategy destined to fail, he said: "There is no way they can get a brick out of this wall."
(Additional reporting by Ece Toksabay; writing by Nick Tattersall; editing by Peter Graff)