BERLIN — Germany's Greens lurched to the left at a party congress in Berlin over the weekend by endorsing a "soak-the-rich" campaign for new taxes, a risky attempt to win power in September's election that upset the party's pragmatist wing.

Ignoring warnings against raising too many taxes at once from their most successful leader, Greens state premier Winfried Kretschmann, the 800 delegates voted to push to raise the top income tax rate to 49 percent from 42 percent and introduce an annual 1.5 percent wealth tax on assets above a million euros.

Greens delegates even booed one of their most respected leaders, Tuebingen mayor Boris Palmer, when he said the tax increases were not balanced — a humiliating slap in the face for the "realo" or pragmatic wing that has long ruled the roost.

The Greens' shift at the Congress that ended on Sunday seemed calculated to woo left-leaning voters and raise the party's chances of returning to power after September's election to end its eight-year stretch in opposition.

The Greens are the world's most successful pro-environment party after ruling Germany with the Social Democrats (SPD) from 1998 to 2005, but are itching to get back into power.

"It's remarkable to me the way the Greens are drifting to the left and how they kicked the shins of some of their most successful leaders," said Thomas Jaeger, a political scientist at Cologne University. "The fundamental, hard-left wing of the party scored a tremendous success at the party congress."

Jaeger said the Greens' leftward move was probably essential if they wanted to return to power with their preferred partners, the SPD. The SPD has struggled in polls and would win just 28 percent while the Greens are currently at 14 percent.

Together the center-left is polling 42 percent, just behind the 44 percent for Chancellor Angela Merkel's conservatives and their Free Democrat (FDP) allies. While neither the center-right nor the center-left will likely win a majority, analysts expect Merkel to form another "grand coalition" with the SPD.

"The Greens realize that if they don't move left, Germany will end up with another 'grand coalition.' But if they go left and win new voters there, they might have enough," said Jaeger.

"For a whole generation of Greens leaders, this is their last chance to get back into power and become ministers. They want it badly," he added.

The Greens' former environment minister Juergen Trittin is 58, former consumer affairs minister Renate Kuenast is 57 and Greens co-chair Claudia Roth is also 57. The prominent trio went all out to push the party toward the left even though Kretschmann and Palmer warned against it.

'NO COALITION WITH CONSERVATIVES'

"The SPD is the only coalition partner that will help us make Germany greener," Trittin said, explicitly distancing himself from previous Greens party talk of keeping options open for a coalition with Merkel's Christian Democrats (CDU) and their Bavarian sister party, the Christian Social Union (CSU).

"We're not going to form a coalition with a band of corrupt amigos like that," Tritten said, referring to tax evasion and corruption scandals that have battered the CSU in recent weeks.

The Greens endorsed a platform calling the SPD their preferred coalition partners but did not rule out other options.

The Greens began as a peacenik ecological movement with a far-left tilt three decades ago, and became famous for their unpredictable and self-destructive party congress battles that could stretch beyond midnight.

But late last year, at their last party congress in Hanover, they made it clear they were no longer set against the idea of a coalition with Merkel.

The party has ruled one traditionally conservative state, Baden-Wuerttemberg, with the SPD as junior partners since 2011 and ruled in Hamburg with the CDU for three years — a so-called "black-green" coalition that gave the Greens respect as a fiscally responsible movement.

The Greens are proud of their newfound clout. But in Berlin that push towards the center was reversed - to the chagrin of Kretschmann, who is the party's only star attraction.

"I think it causes problems when the overall tax burden is too high," Kretschmann said. "I don't think one should raise more than two taxes in any one four-year period in power. We're not going to do anything that hurts the small and medium-sized business. That's why we're so strong in Baden-Wuerttemberg."