As the presidential campaign has gotten nastier, there've been a few eloquent calls for tapping Chris Christie as Veep (e.g. here and here).
That doesn't mean Christie is nasty, it just means he's fully capable of (and quite used to) winning fights that become nasty.
But Christie has a couple big problems that are mutually reinforcing, and that's the key. Other Veepers have multiple problems, but they tend to be separate.
The first problem is that Christie has said, over and over (and as recently as last year) that he's not ready to be president.
The second problem is that he sometimes explodes in a way that most people wouldn't want a president to explode.
Both are weaknesses, and, most damagingly, both call into question his readiness to be president, and thus you have to multiply and not simply add those liabilities.
Other Veep candidates have plenty of problems, but ones that aren't mutually reinforcing.
For example, Rob Portman is a) dull and b) tied to Bush. Those are both liabilities, but they're not mutually reinforcing. They're about different things.
If Portman's weaknesses were in like kind with Christie's, they'd look something like this -- Portman was Bush's Budget Director, and was also a governor who ran up big deficits.
See how much more troubling that hypothetical becomes for Portman?
He could make a pitch he wasn't responsible for Bush's deficits, but what if he ran up deficits of his own as a hypothetical governor? That makes the deficit thing much more problematic for him.
Also, think about Marco Rubio. His biggest weaknesses are that a) he doesn't have much experience and b) he has some questionable political connections.
Once again, those are separate.
But... just think of how much more damaging it'd be if he didn't have much experience AND had a penchant for making dumb gaffes about foreign leaders. That's mutually reinforcing and much more severe than his current set of liabilities.
Now does all this mean I think Christie would be a bad pick?
Not necessarily. It just means that the Romney team should realize that Christie's weaknesses are inexorably-linked and, thus, potentially more likely show themselves in the campaign.