The Republican National Committee should offer to be the official sponsor of a weekly Democratic presidential debate. There would be no better advertisement for President Trump’s re-election. Every time the Democratic presidential contenders gather together, it’s a contest between the merely delusional, the vaguely vindictive and the patently absurd. There are funny moments, like when Andrew Yang risked the ire of the political-correctness police and told us, “I am Asian, so I know a lot of doctors.” There are substantive moments, like when former Vice President Joe Biden reversed himself on China, apparently now agreeing with Mr. Trump that intellectual-property...