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.
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.
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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.
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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.
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