As a lawyer and former college debater, I probably have an inflated view of the importance of the Presidential and Vice Presidential debates. Actually running the government requires a somewhat different skill set from debating well. That said, and with the first Presidential debate less than a week away, debates are useful for at least the following reasons:
1) Especially in a campaign focused on nonsense (lapel pins, lipstick, etc), debates provide an opportunity to draw out policy contrasts. (This is much more true of the general election than the primaries.)
2) Effective communication with the pubic and others is part of the job of the President, and debates will at least expose those candidates who are not minimally qualified for that task (although these candidates are nonetheless elected sometimes).
3) Debates provide an opportunity for journalists or opposing candidates to ask tough questions with follow-ups, thus pushing the candidates off of their prepared talking points. That in turn gives greater sharpness to policy contrasts and can help the public figure out whether the candidate knows the relevant policy areas in detail beyond the executive summary of the briefing book. And that, in turn, is relevant because an under-informed President will either make under-informed decisions or rely excessively on advisers.
The relatively open formats for the 3 Presidential debates look likely to serve these three functions. However, the Vice Presidential debate will not. According to this NY Times story, the VP debates "will have shorter question-and-answer segments than those for the presidential nominees." The story goes on: "McCain advisers said they had been concerned that a loose format could leave Ms. Palin, a relatively inexperienced debater, at a disadvantage and largely on the defensive."
Not mentioned is that the disadvantage of a more open format would hardly be unfair. The McCain explanation is reminiscent of an old story about a lawyer who, upon hearing a damning piece of testimony by a witness, stands and says "objection." The judge asks "what is the basis for your objection?" The lawyer can't think of anything in the rules of evidence, and so finally says, "too damaging to my client."
Posted by Mike Dorf