Hillary Clinton needs a boring running mate
But mostly, the list consists of people it would be hard for most voters to form a strong opinion about in either direction: Sen. Tim Kaine, Rep. Tim Ryan, Housing and Urban Development Secretary Julián Castro.
This makes sense. Clinton needs a running mate who is like wallpaper: Present, but ideally not too noticeable.
This election is shaping up to be a referendum on Donald Trump. Clinton is not very popular, but Trump is much less popular, and she's going to be depending heavily on votes from people who dislike her but feel they must vote for her over Trump.
The last thing she wants to do is pick a VP candidate who will become a pet hate for some of those voters, or who will distract from Trump's self-immolation.
This doesn't just mean she shouldn't pick a left-wing candidate who might scare away moderates. It's also a reason to ignore suggestions that she pick a candidate specifically designed to appeal to moderates (say, former Defense Secretary Robert Gates) who could alienate Sanders supporters and other people on the left flank of the Democratic party.
The safest pick is a candidate who is reliably liberal, but not especially so - ideologically like Clinton herself, but without the high negative ratings. Her ideal running mate is Generic Democrat.
Of course, the pick should also be someone who could be a valuable leader in her administration and would be ready to serve as president if necessary. (That is, nobody like Sarah Palin.) Fortunately, the Democratic Party contains a wide variety of options who are experienced, competent and dull.
Kaine seems like the most apt to me, but if she doesn't want to choose him, there are plenty of other, substantially equivalent options.