President Obama spoke about the Ukraine crisis Friday afternoon, saying, "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine." Mark Wilson/Getty Images hide caption

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In this image from TV, President Obama shakes hands with Cuban President Raul Castro at the memorial service for former South African President Nelson Mandela in Johannesburg on Tuesday. AP hide caption

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Then-Secretary of State Hillary Clinton and Vice President Biden at a White House event in October 2011. A new book says President Obama's aides were then studying whether to replace Biden with the former first lady on the 2012 ticket. Kevin Dietsch /UPI /Landov hide caption

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Steve Inskeep interviews President Obama in the Oval Office on Monday for NPR's Morning Edition. Pete Souza/The White House hide caption

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Children release 95 balloons to mark Nelson Mandela's upcoming birthday outside the Mediclinic Heart Hospital where Mandela is being treated for a recurring lung infection in Pretoria, South Africa. Chip Somodevilla/Getty Images hide caption

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Former President George W. Bush and his successor, President Obama, at the April 25 dedication of Bush's library on the campus of Southern Methodist University in Dallas. Larry W. Smith /EPA /LANDOV hide caption

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A sea of self-motivated individuals or a web of interdependent talents? Both, of course. Jewel Samad/AFP/Getty Images hide caption

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Allan Lichtman, a professor at American University, discusses his 13 keys to a successful election campaign on April 13 in his office in Washington, D.C. Paul J. Richards/AFP/Getty Images hide caption

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