* Merkel's CDU believes succession debate is premature
* Experts say conservatives have to choose an heir soon
* Von der Leyen, McAllister, de Maiziere seen as possible
* Contenders lack CDU backing or record of vote wins
By Stephen Brown and Andreas Rinke
BERLIN, Sept 17 (Reuters) - It is heresy even to pose the question to the chancellor's supporters before Sunday's general election and, if she does win a third term, the answer will remain evasive. But some Germans are asking anyway: who can succeed Angela Merkel?
"Angela the Great", as one magazine dubbed her, looks a near certainty to be re-elected, the main doubt being whether her centre-right coalition gets a new lease of life or she has to govern with the centre-left. Both scenarios are fine with the public, pollsters say, as long as "Angie" stays in charge.
But two recent credible media reports that she will only serve two or three years of a four-year term, denied by Merkel, will make it hard for her conservative Christian Democratic Union (CDU) to keep a lid on succession talk for long.
Three potential heirs are widely touted - Labour Minister Ursula von der Leyen, Lower Saxony's ex-premier David McAllister and Defence Minister Thomas de Maiziere - though all deny any such ambition. None has Merkel's command of the party, or her ability to glide over daily politics and win elections.
Smiling benignly from giant posters or offering reassurance in bland campaign speeches, the popular 59-year-old chancellor leaves it to underlings to debate policy details with struggling Social Democrat (SPD) challenger Peer Steinbrueck.
A majority of Germans say they feel comfortable under her steady, homely leadership, and many are in no hurry to know who comes after her. Party leaders avoid the issue publicly.
"Either Merkel wins the election and there won't be a discussion for a few years, or she loses and the cards will have to be reshuffled completely," said a member of the CDU's top governing board, speaking on condition of anonymity.
Since nobody in the party believes she can lose on Sept. 22, they are confident the debate "won't get going until a year or two after the election", said another senior party member.
But academics who study the CDU, and some party insiders not in the grip of Merkel-mania, believe it would be short-sighted of the party not to start grooming an heir immediately.
Her aides accept succession speculation will be a growing "annoyance factor" in a third term and risks making her a lame-duck well before the 2017 election - meaning it must be quashed.
"Of course there is a debate about the succession," said one long-standing member of the Bundestag lower house, critical of a cult atmosphere where Merkel has become "our only asset".
"What is the CDU without Angela Merkel? That is the pressing question. When we answer that, a candidate may arise who stands for these values," said the lawmaker, who asked not to be named.
"I really think they need to resolve this sooner than 2017," said politics professor Wichard Woyke, adding that historically the CDU has often behaved as a "Kanzlerwahlverein" - a club for electing chancellors - rather than a vehicle for policy ideas.
"There is no discussion of content. This will be a problem for the CDU in the post-Merkel era," he said.
There are echoes of the CDU under Konrad Adenauer in the early 1960s and Helmut Kohl in the '90s - two towering figures whose reluctance to make way for an heir caused turmoil in the party and, in Kohl's case, contributed to defeat by the SPD.
Kohl was Merkel's mentor, which did not stop her putting the knife in when, as deputy CDU leader in 1999, she criticised the former chancellor over a party financing scandal.
That act of ruthlessness by Kohl's innocent-looking "Maedchen" (girl) put her on the path to power that Wichard said was "a strewn with political corpses".
AND THEN THERE WERE THREE...
The careers of CDU "princelings" came to a premature end.
Potential rivals were sidelined to jobs in Europe, left politics or were felled by scandals of vanity or venality. Two ministers fell over plagiarism in academia and Christian Wulff quit as president for taking cheap loans and free hospitality.
This has whittled the succession shortlist down to three Merkel loyalists.
Going by the rule of thumb that the louder politicians deny interest, the keener they are, von der Leyen is top contender. Dubbed "Mother of the Nation" by one paper in reference to her seven children, she likes to say that each generation has its leader and "my generation's chancellor is called Angela Merkel".
Younger than Merkel at 54 and in many ways her antithesis - a ready smile, blue blood, a western CDU pedigree, cosmopolitan upbringing and vast family complete with dogs and ponies - she ranks close behind veteran Finance Minister Wolfgang Schaeuble in terms of popularity. But she can irritate party colleagues.
"The rule is usually that you either have the party behind you or the public," said the CDU board member. "In von der Leyen's case, it's the public."
Her ministerial experience and progressive profile on issues like gender quotas and same-sex couples, on which Merkel sits on the fence, make von der Leyen a compelling candidate. She may switch after the election to a role with more political heft such as parliamentary floor leader, or even foreign minister.
"The CDU would have to be blind not to pick von der Leyen after Merkel," said eminent social historian Hans-Ulrich Wehler.
McAllister was wildly popular with public and party until he narrowly lost Lower Saxony state to the SPD this year.
"'Mac' would have been the best successor, which is why the CDU was so shocked by his defeat," said a senior CDU colleague.
The half-Scot, who cried in defeat, is embarking on an EU career but, aged 42, could return as a contender in 2017.
They don't come more loyal than de Maiziere, Merkel's most trusted lieutenant until a bungled defence contract tarnished his reputation for competence. At 59, he lacks charisma: one party colleague described him as "a quintessential bureaucrat".
Two others mentioned must first prove they can win votes: Volker Bouffier, 61, defends Hesse for the CDU this Sunday in a close contest; Julia Kloeckner, 41, must win Rhineland-Pfalz in 2016 to live up to headlines billing her as "the next Merkel".
Merkel gives no clues except to say "a successor has always been found". The eventual choice may be as improbable as she was - a Protestant woman from former East Germany leading a predominantly Catholic patriarchy from the West.
Her biographer, politics professor Gero Neugebauer, says she may yet try to match Kohl's record 16 years in office by running again in 2017, which would remove the lame duck scenario.
"You know I don't decide on things until they crop up," has been Merkel's enigmatic response to that idea. The message to "wannabes" waiting for the nod is clear: don't hold your breath.