U.S. President Joe Biden sharpened his attacks against Donald Trump on Thursday, delivering his most forceful assertion to date that the former president and Republican front-runner represents an existential threat to the country’s democratic values and institutions. In a speech in the western state of Arizona, Biden charged that Trump holds the “dangerous notion” that he has unchecked power and is above the law. “Trump says the Constitution gave him, quote, the right to do whatever he wants as president, end of quote. I’ve never heard a president say that even in jest,” Biden said. “Not guided by the Constitutionmore