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
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.
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.
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.
Wisconsin Gov. Scott Walker speaks to an overflow crowd in the Mutt's BBQ parking lot in Mauldin, S.C., after more than 500 showed up for his speech in Upstate South Carolina.
Courtesy Walker campaign
Former Florida Gov. Jeb Bush had a tough week with his changing positions on the Iraq War and trying to explain whether he would have authorized it, knowing what we know now.
Ross D. Franklin/AP
In an interview with NPR's Morning Edition host Steve Inskeep, President Obama said Wisconsin Gov. Scott Walker is advocating a "foolish approach" to Iran and that he should "bone up on foreign policy."