There’s good reason to suppose that tonight’s debate will be the least important in terms of how it affects the vote. For one thing, we can expect the audience to be smaller; not only have many Americans heard just about as much of these candidates as they would ever want to hear by now, but the topic is one which is low-ranked by most voters, and besides there’s Monday Night Football and the deciding game of the National League Championship Series (no swing state teams involved!).

Still, with national polling averages very, very, close right now, even small movement could matter. So what should you look for tonight?

* To the extent that reactions by pundits and reporters are driven by expectations, Mitt Romney surely has a significant advantage: expectations for his ability to handle foreign policy must be pretty low, after his botched European trip and his misfire on Libya last week.

* On the other hand: perhaps expectations are low for good reason! Romney’s positions on foreign policy and national security issues are even more vague than his domestic policy positions. He's trapped between a president who is popular on those issues and a dominant faction within his party which advocates unpopular stances. And, yes, he really is quite inexperienced on the broad range of topics which will be discussed tonight. One can argue that low expectations are just pricing in the chances that Romney will run into trouble.

* Generally, a “priming” effect should help Obama apart from how the candidates perform. Obama gets his highest marks from voters on foreign policy, so an event that pushes voters to think about foreign policy should make them more likely to evaluate him positively than if they were primed to think about, say, unemployment.

* How will Romney handle Bin Laden? One solution would be to bring up the death of Bin Laden early and be generous about giving Obama (and the troops and intelligence agents) credit. The real obvious trap that Romney wants to avoid is having Obama drop Bin Laden on him like a ton of bricks at some point, creating a sound bite that plays everywhere for several days. The trick for Romney will be to claim that Obama has been weak while simultaneously treating Bin Laden as old business that won’t matter for the future.

* How will Romney handle his lack of policy substance? Even if he avoids gaffes, there is what you might call a “policy gap” between Romney and Obama that’s real. Romney falls back on vague pronouncements or symbolic words and phrases — “leading from behind”; “apology tour”; “exceptionalism” — rather than carefully thought out positions. The policy gap has already begun to cost him among some experts on domestic issues, but he’s even more vulnerable to charges of vagueness on foreign policy. Will Romney fill in at least a few details — or can he paper over the holes?

Just one note on substance. The topics, at least as reported, appear to be heavily focused on a very small part of the world. I don’t think it’s critical that the debates cover every single issue, but I do hope that if we’re to have a debate about the Middle East and nearby areas from Libya through Pakistan that the candidates will be pushed to talk about torture, detention, and related civil liberties issues.