Former President Bill Clinton plays with balloons onstage at the end of the fourth day of the Democratic National Convention at the Wells Fargo Center in Philadelphia in July. Aaron P. Bernstein/Getty Images hide caption

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Aaron P. Bernstein/Getty Images

Indiana Gov. Mike Pence, the Republican vice presidential nominee, headlined Wednesday's event. But all eyes were on Ted Cruz earlier in the evening for his refusal to endorse Donald Trump. Win McNamee/Getty Images hide caption

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Win McNamee/Getty Images

Democratic presidential candidate Bernie Sanders speaks during a rally in Washington, D.C., on June 9. Although he didn't win the nomination, Sanders' grass-roots fundraising has broken records. Alex Wong/Getty Images hide caption

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Alex Wong/Getty Images

Will The Millions Of People Who Gave Money To Bernie Sanders Give To Democrats?

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Colorado Democrats pack a gym at a Denver high school during caucuses on March 1, 2016. The crowds and long lines have led to calls for the state to move to an open primary system. Marc Piscotty/Getty Images hide caption

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Marc Piscotty/Getty Images

Americans Don't Like Caucuses, But Replacing Them With Primaries Isn't Easy

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