Editor's note: Todd Graham is the director of debate at Southern Illinois University, Carbondale. He has coached his teams to national championships and has been honored with the Ross K. Smith national debate coach of the year award. Graham has analyzed presidential debates for five elections.

(CNN) -- There were many areas of agreement on past policies Monday night in the presidential debate. The problem for Mitt Romney was with the one major disagreement on future foreign policy.

I've always taught my debate students that in any argument, whether at home, at the office, or in public debates, it's important to find areas of agreement.

There are two reasons for this. The first is so that you build up your credibility for when you need it on more important issues. In a debate, if you've already agreed with your opponent on some topics, the audience is more likely to believe you when you finally disagree. And the second reason for agreement is that it lets you pick your battles. Explicitly agreeing with your opponent is a terrific and underused debating strategy that lets you succeed when arguing for bigger, more significant issues.

It was 40 minutes into the foreign policy debate before I heard an area of disagreement on future foreign policy direction. The problem was that Romney had not actually built up his credibility by agreeing with President Obama on any specific future policies. It was painfully obvious. In failing to openly agree when possible, Romney lacked the credibility needed when that first big disagreement came. And when he finally did disagree? Best said by the Holy Grail Knight in the Indiana Jones movie: "He chose -- poorly."

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