ISTANBUL (Reuters) - Immaculately coiffed with a camera-ready smile, Mustafa Sarigul's composure cracks a little as his campaign bus swings towards Istanbul's old city walls.

Two months before elections which could reshape Turkey's political landscape, the main opposition candidate for mayor of its biggest city is taking the fight to a bastion of Prime Minister Tayyip Erdogan's Islamist-rooted AK Party.

At stake is much more than just local politics, and Sarigul's secularist Republican People's Party (CHP) is hoping for a breakthrough in places like the conservative district of Fatih in Istanbul's historic heart.

"We haven't won Fatih for the last 20 years," said Oguz Kaan Salici, the CHP head in Istanbul and one of Sarigul's campaign organisers.

The municipal elections on March 30 will be the first concrete test of Erdogan's popularity since street protests swept Istanbul and other cities last summer, and a corruption scandal last month which forced the resignation of three ministers and opened a feud with an influential Islamic cleric.

His problems have been compounded by a dive in the lira currency, prompting a big emergency interest rate rise this week which is likely to dent economic growth just as Turks vote, tarnishing a reputation for strong financial management.

Istanbul will be a bellwether of whether the turbulence has damaged Erdogan - Turkey's most powerful leader since Mustafa Kemal Ataturk founded the modern republic on the ashes of the Ottoman Empire just over 90 years ago - and if so, by how much.

Good AK Party showings in Istanbul, the capital Ankara and Izmir on the Aegean coast - a stronghold of the CHP - could encourage Erdogan to run for president five months later. AKP officials are targeting 40 percent of the municipal vote, a similar level to the last elections in 2009.

But a drubbing could prompt him to change AK Party rules and stand instead for a fourth term as prime minister. This, Erdogan could declare, would be necessary to save the party he founded, a coalition of conservative religious, centre-right and nationalist elements once accused of endangering Turkey's secular order.

Erdogan's feud with U.S.-based Muslim cleric Fethullah Gulen, who he sees as behind the corruption investigation shaking his government, has damaged the reputation of both, but the AK Party would still win an election if one were called this weekend, a survey showed on Thursday.

The significance of the battle for Istanbul - capital of four empires over the ages and the economic and cultural heart of modern Turkey - is not lost on Erdogan, who served as its mayor and keeps offices in former Ottoman palaces on the shores of the Bosphorus.

Addressing throngs of AK Party supporters a short distance away as Sarigul arrived in Fatih, Erdogan evoked the 15th century conquest of the city - then called Constantinople and under Christian rule - by Muslim Ottomans, in remarks tinged with religious fervour.

"Istanbul is not just a city ... it is the world's pearl, the mother of all cities ... It is not a melting pot for civilisations, it is a city that builds civilisations," he said, a white AK Party scarf draped around his neck.

"We inherited the legacy of a march that has continued for centuries ... Those who do not understand the conquest and the conqueror cannot serve Istanbul," he said to rapturous cheers.

"RESPECTFUL SECULARISM"

Erdogan's AK Party has transformed Turkey over the past decade, ending the period of unstable coalition governments of the 1990s, taming a military that toppled four administrations in the second half of the 20th century, and overseeing a tripling in Turks' nominal wealth.

It has won a growing share of the vote in three successive national elections, trouncing opposition parties too closely wedded to their own segments of Turkish society to mount an effective challenge in all but a handful of regions.

Virtually the whole electoral map - apart from the Aegean coast, the mainly Kurdish southeast and a small region on the European continent west of Istanbul - is under AK Party rule.

But a sense of fatigue is growing, particularly in Western-facing cities such as Istanbul. Many Turks there regard Erdogan's style as pious hectoring and believe he is interfering in their private lives, from trying to impose his hostile views on smoking and alcohol to offering unwanted advice on how many children women should have. Such feelings had been building up long before last summer's demonstrations.

The protests, which drew a heavy-handed police response, brought together groups ranging from anti-capitalist Muslims and gay rights activists to doctors and lawyers. The political opposition may struggle to achieve this diversity at a national level, but Sarigul hopes it could help him in Istanbul.

"There are no 'others' to me. I have always been respectful to all faiths and always defended a secularism that respects everyone," Sarigul told Reuters, eager to demonstrate his party could belie its image as a closed set for the secular elite.

"The CHP is embracing all people, all segments of society," he said, his outing to Fatih on a misty Sunday afternoon emphasising the point.

The CHP's choice of candidate for the district, Sabri Erbakan, aims to appeal to more conservative voters. He is a nephew of former prime minister Necmettin Erbakan, the father of Islamist politics in modern Turkey and of the "Milli Gorus" movement, which seeks to strengthen Islamic values in public life and laid the foundations for the AK Party.

"Fatih is a conservative neighbourhood ... I think my surname is an advantage," Erbakan said, shortly before climbing onto the roof of the campaign bus to be formally presented in a square packed with several thousand supporters.

"I think we should be in unity against recent political developments, that's why I'm here," said the architect and former civil servant, who said his late uncle would have approved. "I will never forget him saying 'Mustafa Bey (Sarigul) is also a son of this country, of course you can serve him'."

Erbakan is in a closely-watched race. The CHP won 30 percent of the vote in Fatih at the last election in 2009, compared with 43 percent for the AKP and 13 percent for the Saadet Party, both of them rooted in Milli Gorus ideology.

DIRTY FIGHT

In a sign of how seriously Erdogan is taking the battle with Sarigul, he used a rally of AK Party faithful in an Istanbul sports stadium to revive old accusations of corruption against the man who could change his own political destiny.

"This is not electronic mail. This is original, original," Erdogan said, brandishing a file labelled "allegations", and showing an enlarged copy of what he said was a CHP report into Sarigul's alleged illegal activities, as the crowd booed.

Sarigul rejects the allegations, which date back several years, as "dishonourable propaganda" and said they were all investigated and dismissed at the time.

Suggestions that followers of Fethullah Gulen could support Sarigul's bid have done little to calm Erdogan's nerves. The prime minister already sees the cleric, whose "Hizmet" (Service) movement exercises deep if covert influence in the police and judiciary, as being behind the corruption scandal he views as a plot to unseat him.

Sarigul declined to be drawn on whether he might benefit from Hizmet support, saying simply that he "enjoys good relations with all communities".

In a political landscape so dominated by one man, Erdogan may also see in Sarigul a charisma that could one day rival his own. The two men appear cut from the same cloth, speaking in crowd-pleasing sound bites and with a sharp eye for a media opportunity.

They also share a similar temperament, quick to anger and eager to control. As Sarigul's campaign bus deviated from the planned route and momentarily lost the convoy, the air turned blue, as he cursed and hammered the glass.

"They don't have to love us ... the more important thing is the attention we receive," the CHP's Salici said, as Sarigul's well-practised grin quickly returned for Fatih's residents, some waving back and cheering, others seemingly bemused as his convoy, horns blaring, snaked its way through the traffic.

(Additional reporting by Ayla Jean Yackley in Istanbul; Writing by Nick Tattersall; editing by David Stamp)