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Social activist Julian Bond, and leader in the American civil rights movement, and Luci Baines Johnson, the younger daughter of President Lyndon Baines Johnson, join hands as they sing "We Shall Overcome" before President Barack Obama spoke at the LBJ Presidential Library, Thursday, April 10, 2014, in Austin, Texas, during the Civil Rights Summit to commemorate the 50th anniversary of the signing of the Civil Rights Act. (AP Photo/Carolyn Kaster) ( Obama Civil Rights )

US First Lady Michelle Obama (L) watches as US President Barack Obama (C) is hugged by Rep. John Lewis (D-GA) at the Lyndon B. Johnson Presidential Library April 10, 2014 in Austin, Texas, during a civil rights summit to celebrate the 50th anniversary of the Civil Rights Act of 1964. (BRENDAN SMIALOWSKI/AFP/Getty Images) ( 528837873 )

The shadow of Air Force One, with President Barack Obama and first lady Michelle Obama aboard, is cast on the ground below as it approaches Austin-Bergstrom International Airport in Austin, Texas, Thursday, April 10, 2014. The president and first lady where en route to the LBJ Presidential Library to attend a Civil Rights Summit to commemorate the 50th anniversary of the signing of the Civil Rights Act. (AP Photo/Carolyn Kaster) ( APTOPIX Obama Civil Rights )

President Barack Obama and first lady Michelle Obama walk to greet supporters on the tarmac after landing aboard Air Force One at Austin-Bergstrom International Airport in Austin, Texas, Thursday, April 10,,2014. The president and first lady traveled to the University of Texas at Austin to attend the Civil Rights Summit, a conference commemorating the 50th anniversary of the 1964 Civil Rights Act. (AP Photo/ Tamir Kalifa) ( Obama Civil Rights )

US President Barack Obama tours the great hall of the Lyndon B. Johnson Presidential Library April 10, 2014 in Austin, Texas. Obama is attending a Civil Rights Summit to celebrate the 50th anniversary of the Civil Rights Act of 1964. (BRENDAN SMIALOWSKI/AFP/Getty Images) ( 528835784 )

U.S. President Barack Obama gives the keynote speech on the third day of the Civil Rights Summit at the LBJ Presidential Library April 10, 2014 in Austin, Texas. The summit marks the 50th anniversary of the passing of the Civil Rights Act legislation. (Photo by Laura Skelding-Pool/Getty Images) ( 483824917 )

Mavis Staples sings "We Shall Overcome" at the Lyndon B. Johnson Presidential Library April 10, 2014 in Austin, Texas. US President Barack Obama is attending the civil rights summit to celebrate the 50th anniversary of the Civil Rights Act of 1964. (BRENDAN SMIALOWSKI/AFP/Getty Images) ( 528838064 )

From left former US Senator Tom Daschle, Lynda Bird Johnson Robb, Chuck Robb, a former Virginia governor and US Senator, and Maria Shriver join others singing "We Shall Overcome" while waiting for US President Barack Obama to speak at the Lyndon B. Johnson Presidential Library April 10, 2014 in Austin, Texas. Obama is attending a Civil Rights Summit to celebrate the 50th anniversary of the Civil Rights Act of 1964. (BRENDAN SMIALOWSKI/AFP/Getty Images) ( 528837960 )

LBJ Presidential Library Director Mark Updegrove, left, speaks to President Barack Obama and first lady Michelle Obama as they tour the Great Hall at the LBJ Presidential Library, Thursday, April 10, 2014, in Austin, Texas, as they attend a Civil Rights Summit to commemorate the 50th anniversary of the signing of the Civil Rights Act. (AP Photo/Carolyn Kaster) ( Obama Civil Rights )

From left, LBJ Presidential Library Director Mark Updegrove, President Barack Obama, first lady Michelle Obama and Rep. John Lewis, D-Ga., tour and exhibit in the Great Hall at the LBJ Presidential Library in Austin, Texas, Thursday, April 10, 2014, as they attend a Civil Rights Summit to commemorate the 50th anniversary of the signing of the Civil Rights Act. (AP Photo/Carolyn Kaster) ( Obama Civil Rights )

US President Barack Obama (2L), US first lady Michelle Obama (2R) and Rep. John Lewis(R) ,D-GA, tour the great hall of the Lyndon B. Johnson Presidential Library with Mark Updegrove, Director of the LBJ Presidential Library, April 10, 2014 in Austin, Texas. Obama is attending a Civil Rights Summit to celebrate the 50th anniversary of the Civil Rights Act of 1964. (BRENDAN SMIALOWSKI/AFP/Getty Images) ( 528835733 )