U.S. presumptive Democratic presidential nominee Senator Barack Obama (D-IL) waves to the audience after speaking at the American Israel Public Affairs Committee (AIPAC) policy conference in Washington June 4, 2008. Obama said on Wednesday Iran posed a serious threat in the Middle East and vowed to stop it from acquiring a nuclear weapon at the pro-Israel lobby group's conference. REUTERS/Jason Reed (UNITED STATES) US PRESIDENTIAL ELECTION CAMPAIGN 2008 (USA)