By Ercan Gurses and Orhan Coskun
ANKARA (Reuters) - Turkey's ruling AK Party is seeking support from the nationalist opposition for constitutional changes to increase President Tayyip Erdogan's powers, but any alliance hinges on whether its veteran leader can fight off a bid to oust him.
Four senior AKP officials told Reuters they expected the Nationalist Movement Party (MHP) to back a proposal that would allow Erdogan to restore - as first step to a presidential system - the party affiliation he had to yield on taking over the existing, largely titular presidency in 2014.
The plan, stopping short of immediately introducing the executive presidency Erdogan wants, was watered down in a bid to win enough support in parliament for a referendum on the necessary constitutional change, the officials said.
"We expect the MHP to support this new proposal. Other parties will not. But if the MHP supports it, the path will be opened to a referendum in September," one of the officials said, declining to be named because negotiations are continuing.
Deputy Prime Minister Numan Kurtulmus said the plan was a second option, and that introducing the full presidential system wanted by Erdogan remained the first priority. His supporters see such change as a guarantee against the fragile coalition governments that hampered Turkey's development in the 1990s.
But Erdogan's opponents see it as a vehicle for his own ambition and fear growing authoritarianism. Opposition parties are unlikely to endorse such a fundamental change to Turkey's system of governance, which would see parliament sidelined.
The AK Party has 317 lawmakers in the 550-seat assembly and needs at least 367 votes to change the constitution directly, or 330 to hold a referendum. The MHP has 40 seats and its support for the AKP's watered-down proposal could allow it to pass.
Under the current constitution, the president must renounce party affiliations and stay impartial. But Erdogan has retained influence over the AKP by dint of his personal popularity since he resigned as prime minister and was elected president in 2014 in the so far frustrated expectation of rapid transition to a full presidential system.
Prime Minister Ahmet Davutoglu announced last week he would step down later this month after an increasingly public rift with Erdogan. His successor as head of the AKP is widely expected to be a staunch Erdogan ally.
The uncertainty, watched with concern by NATO allies, comes at a time when Turkey faces threats from a Kurdish insurgency and spillover from war in Syria. It also coincides with tensions with the European Union over handling of migrants an refugees.
Formalizing Erdogan's grip on the party would allow him to restore full control of Turkey's dominant political movement and further enshrine his hold on government.
"I see what is happening between the current MHP leadership and AKP as going beyond flirting to an engagement," Hakan Bayrakci, head of pollster Sonar, told Reuters.
But the MHP is locked in a leadership battle whose outcome will be key to Erdogan's plans. Several hundred party members have launched a bid to challenge Devlet Bahceli, leader for much of the last two decades, at a special congress set for May 15.
Some might see irony in such a linking of the fates of Erdogan, who virtually swept away the old party order in 2002 elections, and Bahceli who was part of the fractious party politics of the 1990s. But there is common ground between MHP and AKP, both embracing nationalist and conservative religious elements.
The MHP congress 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.
The main candidate to replace Bahceli, should the congress go ahead, is Meral Aksener, a 59-year-old woman who served as interior minister in the 1990s and is seen as unlikely to tolerate MHP members backing the AKP plan.
Pollsters forecast support for the MHP could also double to above 20 percent if she were to lead the party, making the AKP's chances of a stronger parliamentary majority even harder to achieve.
"The MHP is waiting for a decision and the situation will affect support for this (constitutional) measure, the referendum, and the political balances. We're watching their congress process closely," the AKP official said.
The Yargitay appeals court said on Wednesday it would complete its review in May, suggesting the congress is unlikely to be held on time and that the judiciary, over which the government has increased its influence in recent years, is politically divided over the case.
"The MHP could support (proposals for) a party-linked president because they think the government could impact the Yargitay process and be influential in Bahceli keeping his seat," Bayrakci said.
AKP officials rejected any suggestion the government was influencing the court, or that the MHP's leadership battle and AKP efforts to win its support on the constitutional change were in any way linked.
"We have no influence on the court's decision. Such allegations are nonsense. We're not interested in the MHP's internal affairs," a second senior AKP official said.
(Additional reporting by Ece Toksabay; Writing by Daren Butler; Editing by Nick Tattersall and Ralph Boulton)