* Party's political chief to quit amid infighting
* Support for Pirates has shrunk to 3 percent
* Pollsters say unlikely will score in Sept. vote
By Sarah Marsh
BERLIN, March 7 (Reuters) - A leader of Germany's Pirates has said he will resign after infighting tarnished the image of the anti-establishment party that has sunk from being the country's third most popular to a virtual has-been.
The Pirates stormed onto the political stage one and a half years ago, winning 9 percent of the vote in Berlin's assembly and tapping into a deep vein of voter discontent running not only through Germany but across Europe, plagued by debt and weak economies.
The party, which promised to use the Internet to promote more direct democracy, won seats in three more state votes and looked set to shake up Germany's political landscape in September's federal election.
But it failed to hammer out specific policies and has recently become better known for public spats, played out online via often vicious Twitter or blog posts, and a string of resignations.
Johannes Ponader, the Pirates' chief policy maker, told party members on his blog he would step down at the next party conference in May in order to "ease conflicts on the board" and take responsibility for shrivelling popularity.
Ponader last month published a text message exchange that he claims was between himself and Berlin delegate Christopher Lauer, in which the latter urged him to resign.
In a party survey, members said he was "egoistic", "polarising" and "in no way capable of teamwork".
Support for the Pirates has shrunk from 13 percent last year to around 3 percent, well below the 5-percent threshold needed for it to enter parliament at September's federal elections.
The party already failed to clear this hurdle at the most recent regional elections, in Lower Saxony. (Reporting by Sarah Marsh, editing by Gareth Jones and Michael Roddy)