Republican Ted Cruz (left) addresses the media after a campaign rally earlier this month in Kansas City, Mo.; Democrat Bernie Sanders speaks at a town hall event last week in Milwaukee. Polls ahead of Tuesday's Wisconsin primary contests gave Cruz and Sanders narrow leads. Kyle Rivas (L) and Darren Hauck (R)/Getty Images hide caption

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Wisconsin Gov. Scott Walker speaks at a news conference Monday in Madison, Wis., where he announced that he is suspending his Republican presidential campaign. Morry Gash/AP hide caption

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People can now contribute to presidential campaigns with just a few taps on a smartphone. Jonathan Alcorn/AFP/Getty Images hide caption

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#Cashtag: Twitter To Allow Direct Campaign Contributions

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Gov. Scott Walker, R-Wis., holds up a $1 bill during a town hall in Las Vegas where he announced his plan to take on labor unions. Isaac Brekken/AP hide caption

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Scott Walker's Anti-Union Push May Not Prove So Easy As President

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Ohio Gov. John Kasich's supporters have spent more on TV ads than any other presidential candidate, despite a late entry into the race. New Day For America/YouTube hide caption

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In Turnabout, Candidates With Less Spend More, Candidates With More Spend Less

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A man carries a cardboard cutout of Hillary Clinton in Des Moines, Iowa. Clinton and other traditional candidates have struggled to breakthrough as outsiders have captured the attention this summer. Charlie Neibergall/AP hide caption

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Republican presidential candidate and Wisconsin Gov. Scott Walker speaks at the Iowa State Fair, where he encountered protesters over his record in Wisconsin. Charlie Neibergall/AP hide caption

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Republican presidential candidates, from left, Lindsey Graham, Ben Carson, John Kasich, Chris Christie, Bobby Jindal, Jeb Bush, Scott Walker and Rick Santorum speak among themselves after a forum Monday, Aug. 3, 2015, in Manchester, N.H. Jim Cole/AP hide caption

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Wisconsin Gov. Scott Walker frequently invokes his love of the Wisconsin-based discount retailer on the campaign trail as an example of how the government could bring in more revenue from lower taxes. Toby Talbot/AP hide caption

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