Having been knocked from his front-runner perch, a fiery Jeb Bush lashed out at Republican rival Donald Trump on Thursday as the former Florida governor fights to energize his stalled campaign and stop the billionaire businessman’s summer surge. Bush’s name recognition and money—see his recent $100 million-plus fundraising haul—have kept him near the top of the Republican pack, where he has saved his most aggressive criticism for Democrat Hillary Rodham Clinton for much of the year. Now slipping in the polls, Bush abruptly changed course over the last 24 hours, following some Republican competitors who acknowledge Trump has become a...