By Erik Kirschbaum
BERLIN (Reuters) - The leader of Germany's Social Democrats (SPD) took the stage at a Greens party congress on Saturday with an unashamed pitch for them to throw in their lot with the SPD to defeat Chancellor Angela Merkel in September.
It was the first time an SPD leader had addressed a Greens congress. Sigmar Gabriel, whose party would need a coalition with the rising pro-environment party to have any chance of leading the next government, delivered a passionate plea to the Greens to stop flirting with Merkel's conservatives.
He appeared determined to smooth over the tensions this has raised and was remarkably complimentary about a rival only 33 years old that the SPD has sometimes treated as a poor stepsister.
"Our opponents want us to maul each other," Gabriel said. "As in any relationship, you want to look around and see what's available out there before you decide to move into a flat with a new partner.
"There's no harm looking. We all do that," he said, before adding: "You're a special party, you've had a decisive influence on Germany and you've made Germany a better place."
Gabriel said only an SPD-Greens coalition would have the courage and determination to take on financial markets, which he blamed for much of the economic turmoil in Europe and around the world in recent years.
"There are only two parties in Germany that can tame the financial markets, and that's you and us," he said.
The Greens were once a left-wing fringe party with between five and eight percent of the vote. But in the wake of the Fukushima nuclear disaster and a successful CDU-Greens coalition at state level in Hamburg, they are now comfortably Germany's third national party with about 15 percent in opinion polls.
In the conservative state of Baden-Wuerttemberg, the Greens finished ahead of the SPD in a 2011 election and installed their first state premier, Winfried Kretschmann.
While the SPD make a more natural center-left ally for them, many Greens want to keep their options open in the likely event that SPD and Greens combined fall short of a majority in September's election.
Together, they currently trail Merkel's center-left coalition of conservatives and Free Democrats (FDP) by about 44 percent to 42 in surveys. Since neither coalition is likely to win a majority, a CDU-SPD "grand coalition" appears the likeliest outcome.
However, Merkel's conservatives have also reciprocated the Greens' advances as an option to stay in power.
While Merkel remains Germany's most popular politician by far, the FDP has plunged from 14.6 percent in the 2009 election to around 5 percent in polls, suggesting that she may not win a new majority with the FDP.
The SPD, polling around 20 percent, and the Greens campaigned together in 2002 and 2005 - a tactic also employed by the CDU-FDP alliance that can boost both parties' votes.
(Reporting By Erik Kirschbaum; Editing by Kevin Liffey)