Hillary Clinton has as much experience as any Democrat in debating Donald J. Trump.The 2016 presidential campaign, when she was her party’s nominee, included three of the six general-election debates Mr. Trump has participated in. Those faceoffs went a long way toward shaping the country’s vision of his candidacy and what he would be like as president.Mr. Trump, of course, went on to win the 2016 election — an outcome that still haunts Democrats.When Mrs. Clinton called this week to discuss her old debate coach, Karen Dunn, who is helping out Vice President Kamala Harris this time around, I took...