Now that John McCain has finally decided to show up for the debate in Oxford, Mississippi tonight, here are five things that you should keep your eye on while enjoying the debate. Look out for the economy, the sound bite, and temperament among other things.

1). The Economy – It is pretty simple. McCain’s strength is foreign policy and military affairs. His huge weakness is on the economy. McCain won his party’s nomination because of the war in Iraq, but his recent slide in the polls is all about the economy. For McCain to have a strong debate tonight, the emphasis must stay on foreign policy. The more time spent on the economy, the worse McCain looks.

The bar is set low for Obama tonight. He only needs to hold his own against McCain against foreign policy. This will be Obama’s best chance, so far, in the fall campaign to answer voters’ concerns about his experience. For Obama, the key isn’t his positions, but convincing people that he is up to the job. If the line of the debate ends up being about the economy, then Obama is in great shape, but if the Democrat makes the big mistake the door will be open for McCain.

2). Lincoln-Douglas This Isn’t – Neither of these candidates are great debaters. John McCain’s troubles in front of a camera are well documented. He makes George W. Bush look like Ronald Reagan. In the 20 + debates against Hillary Clinton, Obama was mediocre at best. He sometimes struggles to give short clear answers. This debate will not be pretty or be remembered as a stylistic work of art. It has the potential to be a bit personal and ugly. If Obama matches McCain’s expected aggressiveness, this thing will deteriorate quickly.

3).Warm vs. Cool - McCain thrives on conflict, and can get angry fast. Obama was given to flashes of anger during the primary debates, but no outbursts. In 2000, George W. Bush was able to use McCain’s temper against him to his advantage. If Obama gets under McCain’s very thin skin, then the Republican could play into the notion that he is the angry old man. McCain’s temper is ugly, and needs to be kept in control.

In contrast, Obama has shown ability to remain on even keel during the debates. Obama was able to handle whatever Hillary Clinton threw at him, and she is a thousand times more the debater than John McCain could ever hope to be. Obama has been tested in a way that McCain hasn’t. If McCain blows up, and Obama looks presidential, McCain will have a big problem. On the flip side if McCain looks the aggressor, and Obama seems too cool and passive, this will help the Republican.

4). Looking Presidential – This may seem superficial, but many voters make up their minds based not on what the candidates say, but who looks more presidential. The perception battle could be the most important unspoken element of this debate. Obama has seen his margin expand in the polls, because he has looked more presidential than McCain in handling the economic crisis. Tonight is McCain’s chance to get some of that momentum back.

If McCain looks like he is floundering in any way during tonight’s debate, Obama could find himself in a position to run away with this race. McCain could dominate the foreign policy debate, and still hurt himself if he drops the ball on the economic questions. McCain has been unable to look presidential on both foreign and domestic policy during this campaign. If Obama needs to reassure people that he can handle the foreign policy part of the job, McCain needs to do the same on the domestic front.

5). The Sound Bite – The debates are won and lost on sound bites. Who can forget Al Gore’s sighs in 2000, and the way that they played into the Bush campaign’s portrayal of Gore as a know it all? A sound bite from a debate can crystallize an opponent’s campaign theme, and make the difference in a close presidential election. Presidential debates have devolved from gatherings that look to inform voters into zinger fests where each campaign tries to land the sound bite that sticks, and dominates the media coverage for the 24 hours after the debate.

Obama and McCain have already waged a deeply personal and heated campaign against each other. If this debate becomes about personal zingers, then McCain and his non-issues oriented campaign has the advantage, but if McCain is forced to talk about the issues, Obama has an advantage.

I am looking for Obama to try to frame McCain’s worldview as outdated and wrong. This is a subtle way for Obama to make the old age does not equal change argument. Obama will try to make McCain defend his support of Bush. If the Democrat is successful, he could put McCain on the defensive and have himself a very good night, but barring a major gaffe, I doubt that this debate will settle anything.