By Erik Kirschbaum
BERLIN (Reuters) - Once a fringe left-wing movement born in the heat of 1970s radicalism, Germany's Greens party may be heading for the unthinkable - a partnership with the conservatives that would keep Angela Merkel in power.
Greens leaders deny any interest in forming a coalition with the Christian Democrats, but voting patterns and shared views on a surprising number of key policies may push both sides to break a taboo and form a ruling coalition after elections next year.
Having shared power with the Social Democrats (SPD) from 1998 to 2005, the Greens long ago dropped their outsider image and are keen to return to power. A coalition with the main centre-left party would seem once again to be the natural fit.
But, with the SPD losing votes to the Greens and Merkel's Free Democrats (FDP) coalition partner likely to exit parliament entirely, a "black-green" coalition could suit both parties - although neither is saying so, for fear of alienating their core supporters.
"The possibility of a CDU-Greens coalition has never been greater," said Gerd Langguth, a Bonn University political scientist.
"It's not probable but possible. If there isn't enough for a centre-left coalition with the SPD, I'm sure the Greens will change their tune on election night and form a coalition with Merkel. They want to get back into power."
At a more local level, the two parties worked together in a reasonably successful coalition in the city-state of Hamburg from 2008 to 2011. The conservative Baden-Wuerttemberg has been run by a Greens state premier for the last 18 months.
The parties hold similar positions on fiscal discipline, the need for more euro zone integration and energy policy, now that the CDU has agreed to scrap nuclear power by 2022.
But they fiercely disagree on social issues, such as the CDU-backed preferential tax treatment for married couples.
Talk of a black-green coalition - so-called due to the CDU's black party banner - erupted this week after the Greens unexpectedly picked a moderate pragmatist, Katrin Goering-Eckardt, as one of their two lead candidates for 2013.
While the issue is not on the agenda at a three-day Greens party congress in Hanover starting on Friday, it will be on everyone's mind - especially after the Greens' Fritz Kuhn upset a CDU candidate to become mayor of Stuttgart last month.
"A black-green coalition at the federal level is very much a possibility in 2013," said Manfred Guellner, director of the Forsa polling institute. "They speak the same language and have similar values. It was never a likely coalition. But at the same time the odds for 'black-green' were never higher than now."
READY IF MERKEL CALLS
More pressing that the policy convergences is simple coalition arithmetic.
Merkel's conservatives are at 39 percent, according to a Forsa poll on Wednesday but their FDP coalition partner is now at only 4 percent. If it polls less than 5 percent in September, it will win no seats in parliament and Merkel will be forced to find a new partner.
The SPD, which shared power with Merkel from 2005 to 2009 as junior partner is her likely first choice again if the centre right fails to win a majority.
But SPD chancellor candidate Peer Steinbrueck has ruled out serving in another "grand coalition" after the SPD suffered a steep slide in popularity last time. The SPD would like a coalition with the Greens, but with its support down at about 26 percent, that would not secure a majority, even with the Greens polling a high 14 percent.
"It doesn't look like there's going to be enough support for SPD-Greens," said ex Greens leader Daniel Cohn-Bendit. "It's possible voters could force us into another option and we should be ready for that," he told Der Spiegel online.
Greens leaders are reluctant to pledge fealty to the SPD - fuelling speculation they are opening themselves to the CDU.
"You should never say never," said Gero Neugebauer, political scientist at Berlin's Free University. "Neither the Greens nor the CDU would admit this now. But if the Greens have a choice of opposition or 'black-green' after the polls close and the CDU makes an acceptable offer I'm sure they'll take it."
(Editing by Robin Pomeroy)