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Scott Walker the most tested of GOP candidates?

"This is a common thing for candidates to say in this age of cutthroat politics, an effort to show they can withstand challenges once in office and equally important during the general election campaign. But the term is used so much it's not clear how much traction that will give (Walker) as he tries to come back from a summer of being drowned out by Trump," said Zelizer, whose books include a biography of President Jimmy Carter and an examination of the politics of national security.