Ms. Clinton's habitual guardedness is not serving her or the public well. Incidents like the one on Sunday inevitably make her come across as a person with something to hide, and in this case it was totally unnecessary. If this campaign is waged on ideas, experience and temperament, Ms. Clinton wins. She can't afford to allow it to be about anything else. It's telling that polls show the number of voters who say they are still undecided is near record highs for this stage in a race, which reflects the public's general uneasiness with both Ms. Clinton and Mr. Trump. Ms. Clinton clearly can't afford to give her opponent any more ammunition to use against her or let the substantive issues that divide her from Mr. Trump get lost in the media storm over her stumbles.