Earlier on Thursday, I suggested that we have now gotten some pretty good guidance on the size of President Obama’s convention bounce. For the next week or two, most of our attention will be focused on how much of that bounce he maintains — and how it translates into swing states — and not on trying to gauge its magnitude.

However, there has been a significant news event since the conventions, with the killing of four Americans in Libya.

Personally, I find the murky back story about the film that ostensibly prompted the attacks more interesting than its implications for the election, but the attacks do have the potential to complicate polling analysis.

In general, the historical experience is that unexpected foreign policy crises are more likely to move the polls in favor of the incumbent in the immediate term — the rally-around-the-flag effect. However, the effects are much less predictable as the crises play out, with their various diplomatic, military and economic implications.

Even acknowledging that Mitt Romney offered a poorly reviewed initial response to the attacks, I think we’ll need to be a bit careful if there are signs of a shift toward Mr. Obama in the polls because of them.

I don’t mean to suggest that the attacks won’t have any implications for the elections in November, or that we’ll make no effort to analyze what they might be. But it will be important to keep the causality straight. In the event that Mr. Obama appears to gain any further ground in the polls over the next week or so, it may well be improper to describe them as reflecting continued “momentum” from his convention, when the public’s reaction to the Libya attacks may be responsible instead.

Of course, this cuts both ways. Both Mr. Romney’s reaction to the attacks and the news media’s generally negative depiction of it may have been colored by the recent trend in the polls. And as much as an incumbent might seek to cloak himself in the prestige of the presidency, Mr. Obama’s reaction will not necessarily be untainted by political considerations.

The issue is an inconvenient one for Mr. Romney both in its timing — coming just after a difficult convention period for him — and in its substance, since foreign policy is one area in which the polls show a clear advantage for Mr. Obama (as is often the case for an incumbent president).

I’m not going to offer any opinion about Mr. Romney’s reaction to the attacks — which is not to say that I don’t have one. But I think it can be acknowledged that he is potentially operating on an unlevel playing field. His reactions are liable to be judged in a harsher light, both by the news media and probably also by voters, almost no matter what they might be.

And yet, an exceptionally skilled campaign ought to recognize that it is never operating in a vacuum, and that the evaluation of its reaction to news events is likely to be affected by these contingencies. It may have to employ some delicacy in choosing its battles.