Mrs. Clinton has every political reason to avoid wading into the discussion of sexual assault that has riled a nation and thrown her Republican rival’s candidacy into chaos. Not known as a naturally emotive public speaker, Clinton risks stumbling if she embraces the issue at a time when polls show that she is in her strongest position yet to defeat Mr. Trump on Nov. 8. She has played it safe, all but disappearing from the campaign trail until the next debate in Las Vegas on Wednesday.