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An attendee holds a 'Student Loan Justice' sign while waiting for Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, to speak during campaign rally near Robert F. Kennedy Memorial Stadium in Washington, D.C., U.S., on Thursday, June 9, 2016. President Barack Obama formally endorsed Hillary Clinton to succeed him as the next U.S. president today in a bid to unify the party two days after she secured enough delegates to clinch the Democratic nomination. Photographer: Andrew Harrer/Bloomberg via Getty Images

An attendee holds a 'Student Loan Justice' sign while waiting for Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, to speak during campaign rally near Robert F. Kennedy Memorial Stadium in Washington, D.C., U.S., on Thursday, June 9, 2016. President Barack Obama formally endorsed Hillary Clinton to succeed him as the next U.S. president today in a bid to unify the party two days after she secured enough delegates to clinch the Democratic nomination. Photographer: Andrew Harrer/Bloomberg via Getty Images