German Green party members on Sunday defied warnings from their more pragmatic leaders in backing two sharp tax increase proposals, a move that could alienate centrist voters but raise the prospect that a left-wing government can be formed after September’s federal election.

Some eight hundred party delegates gathered in Berlin defied their most successful leader, Baden-Württemberg state premier Winfried Kretschmann, to support a top income tax raise from 42 to 49 percent as well as the introduction of a 1.5 percent wealth tax on assets over €1 million.

Kretschmann had argued in an interview with the Süddeutsche Zeitung that was published on Friday that the combination of the proposed tax hikes would impose an “unreasonable burden” on especially middle income families. He also cautioned his party against ruling out a coalition with Chancellor Angela Merkel’s conservatives whom, he agued, “aren’t that far removed” from the Greens.

Bündnis 90/Die Grünen, once a far-left pacifist movement, has reinvented itself as a more cosmopolitan, socially liberal party in recent years, entering a coalition with the conservatives in Hamburg in 2008 and in Saarland the following year.

In May 2011, the party won its first state prime ministership in the traditional conservative stronghold of Baden-Württemberg where Kretschmann, who has also served as the German parliament’s upper chamber’s president since late last year, leads a coalition government with the Social Democrats, still the senior partner on the left nationwide.

Kretschmann, a former chemist, benefited from Germany’s rising anti-nuclear sentiment in the wake of Japan’s Fukushima power plant disaster. Merkel’s conservatives adopted the Greens’ position to shut all of Germany’s nuclear power plants and subsidize renewable electricity generation instead.

Many Green party members remain unconvinced. Former environment minister Jürgen Trittin said that in spite of her government’s environmental policies, “the failure of Merkel on climate change is reason enough to opt out of her coalition.” Referring to tax evasion and corruption scandals that have battered Merkel’s Bavarian sister party in recent weeks, he added, “We’re not going to form a coalition with a band of corrupt amigos like that.”

Opinion polls have consistently put the Greens at between 14 and 16 percent support since the start of this year, replacing Merkel’s liberal coalition party as the third largest in the country. The Social Democrats, however, are struggling to get more than a quarters of the votes while Merkel polls at over 40 percent, making a “grand coalition” between the two the likeliest outcome of September’s election.

The Greens may calculate that there is little for them to gain in the center whereas a more aggressively leftist program could lure voters away from Die Linke, a socialist party that is increasingly confined to former communist East Germany and polls at around 7 percent nationwide. For the strategy to be successful, moderate leftwingers should vote for the Social Democrats instead and give them a plurality big enough to form a majority government with the Greens.