This was written by CBS News consultant Joe Trippi.
In every single poll John McCain has higher ratings with voters on foreign policy than Barack Obama. So its pretty simple -- if at the end of this debate voters think that Barack Obama deserves to stand on the same stage with John McCain on foreign policy, then hands down John McCain has lost the debate.
This means from the start Barack Obama has a much easier time winning this debate than McCain does.
The fact is that Barack Obama has been answering foreign policy questions for 19 months or more. His sparing with Hillary Clinton has sharpened his debate skills. He knows his answers and it's unlikely that there is any question that he hasn't heard or will cause him to stumble. All of which is likely to make him appear surprisingly knowledgeable and thoughtful to undecided swing voters who have doubts about him on foreign policy and his readiness to be commander in chief.
If Barack Obama goes through the entire debate and holds his own with the "more experienced and knowledgeable" McCain – it helps Obama. It raises Obama's stature on foreign policy issues and more voters are likely to see him as "presidential".
The more he stands with McCain and doesn't stumble, the more it will be Obama who is scoring.
McCain on the other hand has a tougher hurdle – he has to try to "disqualify" Obama.
McCain cannot afford to let Obama gain ground on him on the foreign policy dimension. The McCain campaign has been raising the difference between "talking" and "doing". And I would expect McCain himself to carry this theme into the debate. But disqualifying someone who has had 19 months to fine tune his answers is not going to be easy.
Bottom line? This is the debate that could decide who wins the election and it could decide it in a big way.
If McCain somehow successfully disqualifies or raises serious doubts about Obama on critical foreign policy and defense issues it would be devastating to the Obama candidacy.
But every minute Obama is on that stage with McCain and proves he belongs there, McCain loses his one advantage over Obama and with it the election. Both outcomes are possible, but Obama has the easier job in Mississippi.