Senator Ted Cruz may have urged Donald J. Trump to leave his wife, Heidi, “the hell alone,” but there is one group that is quietly hoping Mr. Trump’s attacks on his rival’s spouse and other women will continue indefinitely: Democrats. As Hillary Clinton turns her attention to a general election campaign, Mr. Trump’s nasty skirmish with Mr. Cruz, including his warning to “spill the beans” about Mrs. Cruz, without offering specifics, and his reposting of a message that mocked her looks, have played into a crucial Democratic strategy to defeat Mr. Trump in November: to portray him as an unabashed...