It has been only three weeks since Hillary Rodham Clinton declared her candidacy for the White House, but she already looks more confident than she did during the almost 17 months of her last campaign. Sure, no serious rival has yet emerged to get under her skin the way Barack Obama did in 2008. But Mrs. Clinton and her team have also shown a determination not to be thrown off course: not by the blowback on her use of personal email while at the State Department, not by reports critical of the Clinton Foundation, nor by congressional investigations of the...