By Stephen Brown and Dave Graham
BERLIN (Reuters) - Rebels in Chancellor Angela Merkel's coalition forced her candidate for the presidency of Germany to a third round of voting Wednesday, in a dramatic display of disenchantment with her leadership.
Merkel desperately needed a decisive victory in the federal assembly that elects the head of state to boost her authority after a series of poor showings in opinion polls and setbacks including the resignation of Horst Koehler as president in May.
For only the third time in post-war history a presidential election went to a third round after Merkel's candidate Christian Wulff failed to win enough votes in a federal assembly where her camp held a comfortable majority on paper.
Analysts said the contest raised doubts about her future after what one called a "slap in the face" for the 55-year-old chancellor, who has run Germany since 2005.
"Maybe someone wanted to send a message to the leadership. Great idea. Wrong day," said Wolfgang Bosbach, deputy leader of Merkel's Christian Democrats in the lower house of parliament.
Candidates only need the most votes to win the third round, rather than the absolute majority required in the first two. That should be a formality because Merkel's centre-right coalition nominally has 644 seats in the 1,244-seat assembly.
But the centre-left's candidate, popular civil rights activist and Protestant Pastor Joachim Gauck, 70, has strong cross-party appeal. A non-partisan figure, he stood up to the communist regime in former East Germany and from 1990 to 2000 ran a commission investigating the Stasi secret police.
Wulff received 600 votes in the first round and 615 in the second, falling short of the 623 he needed. Gauck scored 499 votes in the first ballot and 490 in the second.
"Merkel should fear the twilight of her chancellorship," said the online version of influential weekly magazine Der Spiegel.
Such was the drama surrounding the vote, that hundreds of people gathered outside the Reichstag parliament building to follow events blow-by-blow on giant television screens.
SETTLING SCORES
Merkel has already suffered a string of setbacks this year, with a series of high-level resignations in her party and then Horst Koehler's surprise departure as president in May.
Her nine month-old coalition with the liberal Free Democrats (FDP) looks weak, her ratings in opinion polls are at record lows and her leadership in the euro zone crisis, and recent austerity drive, has been criticised at home and abroad.
She chose an apparently safe candidate to succeed Koehler in the form of 51-year-old Wulff, state premier of Lower Saxony, who was once seen as a future chancellor but ruled himself out in 2008 by saying he was not enough of an "alpha male."
Coalition insiders said the assembly that elects the largely ceremonial president is unpredictable as its delegates are split between members of parliament and representatives from the 16 federal states including show business and sports personalities.
Merkel and her FDP ally Guido Westerwelle have endured weeks of negative publicity, prompting speculation the secret ballot could see the centre right giving vent to its disenchantment.
But the scale of the revolt was a shock, said Gerd Langguth, political scientist at Bonn University: "That was a big slap in the face. I didn't expect that many to vote against her."
The FDP's Development Minister Dirk Niebel said the government candidate would win through in the end.
"There is nothing unusual about some members wanting to settle old scores," said Niebel. "But I can assure you at the end of the day Wulff will be elected president."
Defeat in the presidential contest could even prompt talk of Merkel being replaced, at a moment when Europe is looking to Germany to help lead it out of recession and financial crisis.
(Additional reporting by Erik Kirschbaum and Paul Carrel; Writing by Stephen Brown; Editing by Paul Taylor and Jon Hemming)
