Vice President Hubert Humphrey (right) and his running mate, Sen. Edmund S. Muskie, wave to the crowd with their wives at the Democratic Convention in Chicago on Aug. 29, 1968. Violence ensued inside and outside the convention hall. AP hide caption

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AP

Republican presidential candidate Donald Trump speaks at a campaign rally Monday in Wilkes-Barre, Pa. Trump won Tuesday's primary race in the state Mel Evans/AP hide caption

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Mel Evans/AP

In a Republican race marked by anger and anti-establishment fervor, Kasich is running the optimistic campaign. Is that enough to win? Andrew Burton/Getty Images hide caption

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Balloons fall at the 2012 Republican National Convention in Tampa, Fla. This year's convention in Cleveland is likely to be far more complicated. Charles Dharapak/AP hide caption

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Charles Dharapak/AP

Can A Candidate Pay Delegates? Plus, 3 Possible GOP Convention Scenarios

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Donald Trump takes part in a CNN town hall event in Milwaukee on Tuesday. Darren Hauck/Getty Images hide caption

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Darren Hauck/Getty Images

Why Donald Trump Reversed His Pledge To Support The GOP Nominee

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Clockwise from top left: Hillary Clinton, Donald Trump, John Kasich, Ted Cruz and Bernie Sander Justin Sullivan; Andrew Harrer; Ralph Freso; Bob Levey; Brendan Smialowski/Getty Images hide caption

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Justin Sullivan; Andrew Harrer; Ralph Freso; Bob Levey; Brendan Smialowski/Getty Images

Listen: Hillary Responds To Critics Allegations

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A polling station in Oakmont, Pa., on Nov. 6, 2012. Winning a presidential nomination includes mastering complex rules about delegates, and Pennsylvania's rules are among the most complicated. Jeff Swensen/Getty Images hide caption

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In This GOP Primary, You Can Win The State But Get Only A Few Delegates

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Bernie Sanders addresses a rally at Macomb Community College in Warren, Mich. Sanders needs a good showing in Michigan on Tuesday to start to turn his campaign around. Scott Olson/Getty Images hide caption

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Marco Rubio and Donald Trump participate in the Republican presidential debate in Detroit. Geoff Robins/AFP/Getty Images hide caption

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Geoff Robins/AFP/Getty Images

Marco Rubio Defends His 'Small Hands' Criticism Of Donald Trump

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Donald Trump addresses a crowd in South Carolina. Trump has the clearest path to the GOP nomination, but Republicans opposed to him aren't giving up the fight. Spencer Platt/Getty Images hide caption

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Donald Trump greets supporters after speaking at a caucus night watch party at the Treasure Island Hotel & Casino in Las Vegas. Trump is favored to do well Super Tuesday. Ethan Miller/Getty Images hide caption

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Ethan Miller/Getty Images