By Erik Kirschbaum
BERLIN (Reuters) - Germany's two main opposition parties traded warnings on Sunday against joining forces with Chancellor Angela Merkel's conservatives after September's election if they fail to win their own left-of-center majority.
The leaders of the Social Democrats (SPD) and Greens party issued unusually shrill messages to each others' supporters about the risk their votes might end up going to a party that could join forces in a coalition with Merkel.
The SPD and Greens want to form a center-left government after September's election but opinion polls show they will fall short of the needed margin. Surveys show Merkel's best chances of serving a third term could be to lure either the SPD or the Greens into a coalition with her Christian Democrats (CDU).
SPD chairman Sigmar Gabriel, alarmed about flirtations between the Greens and CDU, told the Welt am Sonntag newspaper that Greens voters should be aware that the pro-environmental party could end in bed with the CDU if the SPD and Greens failed to achieve a majority on September 22.
"That can't be ruled out and Greens voters should know about that," said Gabriel. The Greens, he said, had turned into Germany's "new liberal party" as they were chasing voters who had earlier backed the CDU and pro-business Free Democrats (FDP).
"There is a lot of overlap between the CDU and the Greens now," Gabriel said, trying to deter hard-core Greens voters who have traditionally viewed the conservatives as the party's arch political enemy.
'HOT AIR MERCHANT'
Greens co-chairman Cem Oezdemir, one of the party's most eloquent proponents of an opening to the CDU, quickly shot back.
"Gabriel is a hot air merchant," Oezdemir told Die Welt newspaper. "He knows that we want to defeat the center-right government together with the SPD. But it's not enough to rely only on SPD-Greens voters. If we were to do that, the SPD would quickly end up turning to Merkel for another 'grand coalition'."
Opinion polls show neither Merkel's ruling center-right coalition nor the center-left opposition command enough support to win a majority to lead Europe's biggest economy.
An Emnid poll in Bild am Sonntag showed Merkel's conservatives at 39 percent and their Free Democrat allies at five percent for a combined 44 percent. The SPD was at 26 percent and the Greens at 15 percent for a total of 41 percent.
The SPD and Greens governed together in a coalition from 1998 to 2005. After that the SPD joined forces with Merkel as junior partners in an awkward "grand coalition" until 2009.
The Greens' support in polls has doubled in recent years to levels around 15 percent thanks in part to fears about nuclear power in the wake of the Fukushima nuclear disaster, but also their growing appeal to conservative and rural voters.
The Greens, once famous for their unpredictable and self-destructive party congress battles, have become a serious and united party eager to return to power.
At the state level, the Greens ruled in a harmonious coalition with the CDU in Hamburg for three years until 2011, earning them national respectability as a fiscally responsible party. They have also ruled the state of Baden-Wuerttemberg with the SPD as their junior coalition partners since 2012.
(Editing by Mark Heinrich)