The scandal du jour involving Hillary Clinton — that she used her private e-mail account to conduct official business as secretary of state, possibly in violation of federal law — may well pass. But the uproar, and the fact that it caught so many Democrats off guard, ought to serve as a warning to the party about the dangers of letting Clinton march to the presidential nomination next year without serious opposition. Clinton needs credible opponents, both to keep her on her toes, and so that voters will have real choices and a backup if she falters.

So far, only one candidate, one-term former Virginia senator James Webb, has officially entered the race, but he lacks much of a national profile. Other rumored Democratic candidates are Vermont Senator Bernie Sanders, who is not even technically a Democrat, and the low-key Martin O'Malley, the former governor of Maryland. While Vice President Joe Biden has kept his options open, it seems increasingly unlikely he will run. Many left-leaning groups have urged Massachusetts Senator Elizabeth Warren to challenge Clinton, but she has shown no interest.

Compare that to the Republican side, where a relatively wide and ideologically diverse field appears to be shaping up. GOP voters will likely get a chance to choose not just a candidate, but to weigh in on the direction of their party. Jeb Bush favors immigration reform; Ted Cruz does not. Lindsey Graham generally supports muscular assertion of American military power abroad; Rand Paul generally doesn't. Whoever wins the Republican nomination not only will have been toughened up in the primaries, but will have been forced to engage in a debate over serious questions.