That debates have little effect is the case most of the time. Despite the Obama's poor performance and Romney's more aggressive style, the debate may give the challenger a bump, but it will probably not have that much impact. A bump in the first debate for the challenger is a matter of fact. It's known as the "incumbent's curse." Since the first televised debate, most agree that only three or four depending on who you believe, might be game changers. How much is actually lore and myth or how much an action in a debate was the final brick in an already struggling campaign is probably more the case. Nevertheless, here they are.
There is also much discussion about this being more myth than game changer since the black and white debate really doesn't seem to show that much. Nixon was a master politician. There is still much wonder that he made such a mistake. He changed the course of election history as Vice President candidate for Eisenhower when he gave the famous "Checker's Speech"* which would eventually lead to the the whole thing about candidates releasing taxes in the late '60s, when George Romney released a dozen tax returns.
the statement* played into all this image, especially the idea of not being smart enough to be POTUS.
If you actually look at it, debates that change the elections are more the last straws and something of an anomaly. There are some truly great moments and lines, but few changed the outcome of an election because for the most part many folks already know how they will vote and they will most likely stilt the debate to their own points of view. Some might argue that other debates have had more impact, but there are also many where the first debates winner closed the gap following the debate and still lost.
*link to YouTube video showing the moments. The Checkers speech is in two parts.