In the Hall of Remembrance at the U.S. Holocaust Memorial Museum, visitors can light candles in memory of the 6 million Jews killed by the Nazis and their collaborators during World War II. Nicholas Kamm /AFP/Getty Images hide caption

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President Obama, former presidents George W. Bush, Bill Clinton, George H.W. Bush and Jimmy Carter attend the opening ceremony of the George W. Bush Presidential Center on Thursday in Dallas, Texas. The Bush library, which is located on the campus of Southern Methodist University, with more than 70 million pages of paper records, 43,000 artifacts, 200 million emails and four million photographs. Alex Wong/Getty Images hide caption

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Former President Bill Clinton (and then-Vice President Al Gore) in 1996, the year Clinton signed the Defense of Marriage Act. Stephen Jaffe /Reuters /Landov hide caption

itoggle caption Stephen Jaffe /Reuters /Landov

President Barack Obama joins Former President Bill Clinton on stage during the Democratic National Convention in Charlotte, N.C., on Wednesday. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP

It may be in former President Bill Clinton's (and his wife's) interest to help keep the Democratic party together for the next convention. Carolyn Kaster/AP hide caption

itoggle caption Carolyn Kaster/AP

V ice President Joe Biden, left, and President Obama embrace following Biden's vice-presidential nomination acceptance speech at the Democratic National Convention in Denver, Wednesday, Aug. 27, 2008. Paul Sancya/AP hide caption

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