Why is the Democratic field aside from Hillary Clinton so weak?

Will Wilkinson opines:

There was an air of inevitability about Mrs Clinton early on in the 2008 race, too. However, Barack Obama was, and is, a rare political talent. If anyone comparably gifted is perched on the Democratic bench, his or her light has been kept well hidden. Democrats like to cast Republicans as out-of-touch fuddy-duddies, but the Democratic field, as it now stands, is remarkably long in the tooth, with an average age of 69. The GOP field averages a relatively youthful 57. Where are the Democrats in their avid middle years longing to play on a national stage, labouring now to lay the groundwork for a big run down the road? When the payoff is huge, it can make sense to play even when the odds are slim. Ted Cruz knows he probably won't win this year, but he is bold enough to give it a shot. Why so little intrepid ambition among the Democrats? ...

Despite their recent losses in Congress and in the states, the presidential electoral map remains in the Democrats' favour. Why would a party one election away from utter, catastrophic defeat gamble on anyone less unimpeachably solid? For Democrats, this is no time for romance. You may thrill in your heart of hearts to [Elizabeth] Warren's polemics against the plutocrats, but if no one but Mrs Clinton seems so certain to withstand the Republican onslaught, you may reasonably wish Mrs Warren, and others with their eyes on the prize, to sit this one out.

But is Mrs Clinton really such a safe bet? She struggled with a concussion and blood clots in 2012. What if something like that happens again? In any case, she is not as spry as she once was. She and Mrs Warren are only a few months apart in age, yet Mrs Warren seems markedly younger and more reliably energetic. It's not nice, but such considerations matter in politics. A cakewalk in the primaries risks leaving vulnerabilities unexposed and unfortified.

It's also worth noting that the Democrats' electoral advantage at the presidential level is not a sure thing. It materialises only if the party machine succeeds in getting young, poor and minority voters to the polls. Mr Obama beat Mrs Clinton from her left, and went on to beat John McCain by exciting sometimes tough-to-reach Democratic constituencies. Mrs Clinton's gender is certainly a source of excitement, but her presidency would mark a shift to the right for Democrats at a time when the party's energy is coming from the left. A competitive primary pitting Mrs Clinton against an attractive progressive rising star or two would test whether she remains capable of generating real enthusiasm across the party's varied base. It seems like a test worth running.

Democrats ought to worry at least a little about the possibility that Hillary Clinton has become the contemporary Democratic version of Bob Dole in 1996: an elder statesman, a presumed nominee, universally admired and, when it really counted, insufficiently voted for.