BERLIN (AFP) - Germany's economy minister and vice chancellor Philipp Roesler offered Monday to step down as head of the Free Democrats (FDP), junior partners in the ruling coalition, a party source said."He is ready to step aside," the source said, if his chief rival in the party, Rainer Bruederle, wants to lead the party into the September general election after a bitter defeat for the ruling centre-right in a regional vote in Lower Saxony Sunday.Roesler, who has led the party for nearly two years, has been under fire for months due to aenemic poll ratings and a failure to sharpen the FDP's profile against their partners, Chancellor Angela Merkel's Christian Democrats (CDU).The pro-business FDP managed to capture nearly 10 percent in the cliffhanger state election - more than doubling many pollsters' forecasts and tallying their best result in Lower Saxony in post-war history.But their success came at their allies' expense. The CDU scored just 36 percent, with voters splitting their ballots under Germany's two-vote system in a bid to rescue the state coalition by helping the weak FDP.Around 101,000 voters who backed the conservatives in 2008 plumped for the FDP this time, exit polls showed.The chancellor had campaigned hard for state premier David McAllister, a half-Scot seen as a potential Merkel successor, and his debacle amounted to a political "nightmare" for her, German media said.Merkel, who nevertheless enjoys record popularity in Germany and a solid lead in opinion polls, has stated that she hopes to continue the tie-up with the FDP after the federal election.But pundits say that a "grand coalition" with the Social Democrats appears far more likely at this stage.