BERLIN - Germany's opposition Social Democrats (SPD) and Greens have gained momentum after winning a regional vote last week, a poll showed on Wednesday, giving them a much-needed boost in their bid to oust Chancellor Angela Merkel in September's election.The Forsa poll put the SPD up two points at 25 percent and the Greens up 1 point at 15 percent. Merkel's conservatives dropped 2 points but are still the biggest force at 40 percent.However, her Free Democrat (FDP) coalition partners were flat at 4 percent, one point below the threshold needed to enter parliament.