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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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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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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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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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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Bernie Sanders and Donald Trump deliver victory speeches at their respective watch parties in New Hampshire. Jewel Samad/AFP; Joe Raedle/Getty Images hide caption

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Republican Chris Christie of New Jersey has invested nearly his entire campaign on New Hampshire. A poor result will almost certainly effectively end his candidacy. Robert F. Bukaty/AP hide caption

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An American soldier carrying equipment walks past an American flag at Incirlik Air Base in Adana, Turkey, as U.S. Air Force war planes are used against ISIS. Anadolu Agency/Getty Images hide caption

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